It is narrated from Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) that a man said: O Messenger of Allah, we have come to know how to send salam upon you, so how should we send blessings (salah) upon you? He (peace and blessings be upon him) said: "Say: 'Allahumma salli ala Muhammadin wa ala ali Muhammadin kama sallayta ala Ibrahima innaka Hamidun Majid. Allahumma barik ala Muhammadin wa ala ali Muhammadin kama barakta ala Ibrahima innaka Hamidun Majid' (O Allah, send Your mercy upon Muhammad and the family of Muhammad as You sent Your mercy upon Ibrahim; indeed You are Praiseworthy, Glorious. O Allah, send Your blessings upon Muhammad and the family of Muhammad as You sent Your blessings upon Ibrahim; indeed You are Praiseworthy, Glorious)."
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16390
Sayyiduna Ka'b bin Ujrah (may Allah be pleased with him) said: We asked the Messenger of Allah (peace and blessings be upon him): How should we send blessings upon you? Then the narrator narrated a hadith similar to it.
Hadith Referenceالمعجم الكبير للطبراني / باب الكاف / 16391