Sahih Bukhari Volume 8, Book 75, Hadith Number 369.

Narated By Abu Sa’id Al-Khudri : We said, “O Allah’s Apostle This is (i.e. we know) the greeting to you; will you tell us how to send Salat on you?” He said, “Say: ‘Allahumma Salli ‘ala Muhammadin ‘abdika wa rasulika kama sal-laita ‘ala Ibrahima wa barik ‘ala Muhammadin wa all Muhammadin kama barakta ‘ala Ibrahima wa Ali Ibrahim.”