Sayyiduna Uqbah bin Amr (may Allah be pleased with him) said: A man came to the Messenger of Allah (peace and blessings be upon him) and sat down in front of him (peace and blessings be upon him), and said: O Messenger of Allah, we already know the way of the salam (greeting), but tell us about the salah (blessings): how should we send blessings upon you? At this the Messenger of Allah (peace and blessings be upon him) became so angry that we wished the man had not asked this question. Then he (peace and blessings be upon him) said: "When you send blessings upon me, say: O Allah, send Your mercy upon Muhammad, the unlettered Prophet, and upon the family of Muhammad as You sent it upon Ibrahim and the family of Ibrahim; indeed You are Praiseworthy, Glorious."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14770