Sahih Bukhari Volume 008, Book 075, Hadith Number 368.

Narated By 'Abdur-Rahman bin Abi Laila : Ka'b bin 'Ujra met me and said, "Shall I give you a present? Once the Prophet came to us and we said, 'O Allah's Apostle ! We know how to greet you; but how to send 'Salat' upon you? He said, 'Say: Allahumma Salli ala Muhammadin wa 'ala Ali Muhammadin, kama sal-laita 'ala all Ibrahima innaka Hamidun Majid. Allahumma barik 'ala Muhammadin wa 'ala all Muhammadin, kama barakta 'ala all Ibrahima, innaka Hamidun Majid."