Narrated Ka’b bin Ujra:
It was said, "O Allah’s Apostle! We know how to greet you, but how to invoke Allah for you?”The Prophet said, "Say: O Allah! Send your blessings/greetings (Salawat) on Muhammad and his family, the same way as You sent Your blessings/greetings on Abraham’s family. You are indeed worthy of all praise, full of glory.’"
Sahih al-Bukhari Hadith: 6.321
Narrated Abu Said al-Khudri:
We said, "O Allah’s Apostle! (We know) this greeting (to you) but how shall we invoke Allah for you?”He said, "Say: O Allah! Send your greetings on Muhammad who is Your servant and Your Messenger, the same way as You sent Your greetings on Abraham’s family. And send Your blessings on Muhammad and on Muhammad’s family, the same way as You sent Your blessings on Abraham’s family."
Sahih al-Bukhari Hadith: 6.322
Narrated Ibn Abi Hazim and Ad-Darawardi:
Yazid reported (similarly with the following wordings) "...And send Your blessings on Muhammad and on Muhammad’s family, the same way as You sent Your blessings on Abraham and on Abraham’s family."
Sahih al-Bukhari Hadith: 8.368
Two more traditions are as follows:
Abu Masud Badri relates: (Once) we, were sitting with Sad bin Ubadah when the Holy Prophet came to us. Bashir Ibn Sad asked the Holy Prophet: O Messenger of Allah, we have been commanded by Allah to invoke blessings on you by reciting Salat, then how shall we do this ? The Holy Prophet kept quite for sometime, so much so that we wished that Bashar bin Sad had not asked the question from the Holy Prophet. After some time the Holy Prophet said these words:
O Allah, send Thy blessings on Muhammad and on the children of Muhammad as Thou didst send Thy blessings on Abraham, and bless Muhammad and the children of Muhammad as Thou didst bless Abraham. Indeed Thou art the praiseworthy, the Glorious. Continuing, the Holy Prophet said: and the salutation is already known to you.
Riyadh al-Saliheen, by al-Nawawi, English version, Tradition #1406