(56) اِنَّ اللَّهَ وَمَلآَئِكَتَهُ يُصَلُّونَ عَلَي النَّبِيِّ يَآ أَيُّهَا الَّذِينَ ءَامَنُوا صَلُّوا عَلَيْهِ وَسَلّـِمُوا تَسْلِيماً
56. "Verily Allah and His angels send blessings on the Prophet, O you who believe! Send you also blessings on him, and you salute him with submission.
In the whole six narration books of the Sunnites there are some narrations indicating that once the Prophet (p.b.u.h.) was asked: "How should we send blessings?" He answered: "Say: 'O' Allah! Bless Muhammad and his descendents'."
It is important to recite Salawāt beside the name of the Prophet (p.b.u.h.). The Messenger of Allah said: "Whoever sends blessing on me in his book and his writing(s), he will be rewarded as long as that Salawāt exists in that writing."
Some Islamic narrations indicate: whoever once sends blessings on Muhammad, Allah will send blessings on him ten times and (also) his ten faults will be concealed.
After the statements mentioned in the previous holy verses, about the protection of the Prophet's respect and that he should not be annoyed, the verses under discussion at first speak about the special love of Allah and His angels to the Prophet (p.b.u.h.) and then He commands the believers in this regard. Then, the Holy Qur'ān explains the evil and painful sequels for those who annoy the Prophet (p.b.u.h.) and finally it mentions the great sin of those who hurt the believers by means of slander.
 Sahīh-i-Bukhārī, Tradition No. 5880  The Commentary of Rauh-ul-Bayān following the verse  Majma'-ul-Bayān, the commentary
At first, it says:
on the Prophet, …"
The rank of the Prophet (p.b.u.h.) is so high that Allah, the Creator of the world, and all His angels, who are divinely ordered to manage this world, send blessings on him (p.b.u.h.). Now that it is so, you may come into agreement with this message of the world of existence, as the verse continues saying:
and you salute him with submission.
The Prophet (p.b.u.h.) is an excellent being of the world of creation. Now that, by the grace of Allah, he is available for you, be careful not to count him cheap. Behold not to forget his rank before Allah and with the angels of all heavens. He is a human being. He has come out from among you, but he is not an ordinary man, he is one in whose entity a world can be found.
A Few Points:
1- When the Arabic word /salāt/ and the word /salawāt/, which is its plural form, are attributed to Allah, they will be meant: 'sending Mercy', and when they are attributed to the angels and the believers they are in the sense of 'asking for mercy'.
2- The application of the term /yusallūn/, which is a verb in the present tensed, is an indication to the continuance of the verb; that is, Allah and His angels continuously send blessings and Mercy on him, a permanent mercy and salutation.
3- Commentators have delivered different attitudes about the difference between the Qur'ānic term /sallū/ and /sallimū/ among them that which seems more suitable with the lexical
form of these two Arabic terms is that: the term /sallū/ is an imperative form of the verb that means to seek mercy and to send blessings on the Prophet (p.b.u.h.), while the term /sallimū/ either means to submit to the commandments of the Prophet of Islam (p.b.u.h.) as Sura An-Nisā', No. 4, verse 65 concerning the believers says: "… Then they find within themselves no dislike of which you decide and submit with full submission."
There is a narration which indicates that once Abūbasīr said to Imam Sādiq (a.s.) that he had understood the meaning of sending blessing on the Prophet (p.b.u.h.) but what was the meaning of submitting to him. The Imam answered him: "The purpose of it is being submitted to him in affairs."
Or it means to send salutation to the Prophet (p.b.u.h.) as it is said: "Peace be upon you O' Messenger of Allah!", and the like, the content of which is asking safety and health for the Prophet (p.b.u.h.) from the presence of Allah.
Abū-Hamzah Thumālī narrates from one of the friends of the Prophet (p.b.u.h.) by the name of Ka'b as follows: "When the above verse was revealed we told him we knew the meaning of 'peace be upon you', but how was to send blessing on you?" The Prophet (p.b.u.h.) answered: "Do say: 'O' Allah! Bless Muhammad and his descendents as You blessed Abraham, verily You are Praised, the Glorious. And send blessings on Muhammad and his descendents as you sent blessings on Abraham and the progeny of Abraham; verily you are Praised, the Glorious." Both the meaning of salutation and the manner of sending blessing on the Prophet are made clear from this tradition.
 Majma'-ul-Bayān, following the verse under discussion  Ibid
These two meanings are completely different for salutation, but by proper care they can be lead to a single point, and it is a verbal and practical submission to the Prophet (p.b.u.h.); since he, who sends blessings on him and asks Allah safety for him, loves him and accepts him as a Divine Prophet necessary to be obeyed.
4- It is noteworthy that in numerous narrations recorded by the Sunnites and narrated from Ahl-ul-Bayt (a.s.) concerning the modality of sending blessings on the Prophet (p.b.u.h.), it has explicitly been mentioned that the 'descendents of' Muhammad should be added at the time of sending blessings on Muhammad.
It is mentioned in Durr-ul-Manthūr from Sahīh Bukhārī, and Muslim, Abū-Dāwūd, Tarmathī, Nisā'ī, Ibn-i-Mājah, Ibn-i-Marduwayh, and some other narrators who have narrated from ka'b-ibn-'Ujrah that a man told the Prophet (p.b.u.h.): "We know how to salute to you, but how should be sending blessings on you?" The Prophet (p.b.u.h.) said: "Do say: 'O' Allah! Bless Muhammad and his descendents as You blessed Abraham; verily You are Praised, the Glorious. And send blessings on Muhammad and his descendents as You sent blessings on Abraham and the progeny of Abraham; verily You are Praised, the Glorious." Besides the abovementioned tradition, the writer of Durr-ul-Manthūr have recorded eighteen more traditions in all of which this fact has been stipulated that /'āl-i-Muhammad/ (the progeny of Muhammad) must be mentioned at the time of reciting Salawāt.
These traditions have been taken from the famous books of the Sunnites narrated from a group of the Companions, including Ibn-i-'Abbās, Talhah, Ibn-i-Mas'ūd, 'Abū Sa'īd-Khidrī, 'Abū-Hurayrah, 'Abū Mas'ūd-i-'Ansārī, Barīdah, Ka'b-ibn-'Ujrah, and Amir-ul-Mu'mineen Ali (a.s.).
Sahīh Bukhārī, which is one of the most famous sources of traditions of the Sunnites, has delivered numerous narrations in this regard. For more details you can refer to the text of the book itself.
There are also two narrations recorded in Sahīh-i-Muslim in this regard. 
This point is also noteworthy that in some narrations of the Sunnite and many narrations of Shi'ites even the Arabic word /'alā/ (upon) has not occurred as a separation between 'Muhammad' and 'Āl-i-Muhammad', but the phrase is recited as this: /'allāhumma salli 'alā Muhammad-in wa-'āl-i-muhammad/. We conclude this discussion with another tradition from the Prophet of Islam (p.b.u.h.).
Ibn-i-Hajar cites in 'Sawa'iq' that the Prophet (p.b.u.h.) said: "Do not send an imperfect Salawāt on me" They asked: "What is an imperfect Salawāt?" He answered: "That you say: 'O' Allah! Send blessings on Muhammad' and you do not continue it. But you do say: 'O' Allah! Send blessings on Muhammad and the descendents of Muhammad'."
It is for this very narration that a group of the great jurisprudents of the Sunnites consider incumbent the phrase /'āl-i-muhammad/ to be added to his name in the prayer.
5- Is sending Salawāt on the Prophet (p.b.u.h.) incumbent, or not? And if it is obligatory, where is it? This is a question which the jurisprudents answer.
All the jurisprudents of Ahl-ul-Bayt say that its recitation in the first and second 'Tashahhud' is obligatory, and in other than that it is recommended. Besides the traditions narrated
 Sahīh-i-Bukhārī, Vol. 6, P. 151  Sahīh-i-Muslim, Vol. 1, P. 305  Sawā'iq, P. 144  'Allāmah Hillī, has narrated this saying in Tathkirah, from the scholars of Shī'ah added to Imam Ahmad and some Shāfi'iyyah
from Ahl-ul-Bayt (a.s.) in this regard, there are also a great deal of proper narration recorded in the books of the Sunnites indicating that it is incumbent.
Among them is the famous narration that 'Āyishah narrates. She says: "I heard from the Messenger of Allah who said: 'A prayer without purification and without sending Salawāt on me will not be accepted'."
Among the jurisprudents of the Sunnites, Shāfi'ī says it is obligatory to be recited in the second Tashahhud. Ahmad considers it as obligatory in one of two narrations quoted by him; as well as another group of the jurisprudents. But some others, like 'Abū-Hanīfah, have not counted it obligatory.
It is interesting that Shāfi'ī has explicitly expressed this very pronouncement in his famous poem, where he says: "O' Ahl-ul-Bayt of the Messenger of Allah! The love of you has been enjoined from the side of Allah in the Qur'ān. And for the glory of you, this is enough that whoever does not send Salawāt on you his prayer is invalid."
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 Tathkirah, by 'Allāmah, Vol. 1, P. 126