It is narrated from Sayyiduna Zayd bin Kharijah al-Ansari (may Allah be pleased with him), who said: We said: "O Messenger of Allah! We have come to know how to send salam upon you, but how should we send blessings (salah) upon you?" He said: "Say: O Allah, send down blessings upon Muhammad (peace and blessings be upon him) and the family of Muhammad, as You sent down blessings upon Ibrahim (peace be upon him) and the family of Ibrahim; indeed You are Praiseworthy, Glorious."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 5124