It is narrated from Sayyiduna Abu Mas'ud al-Ansari, Uqbah bin Amr (may Allah be pleased with him), that a man came and sat down in front of the Messenger of Allah (peace and blessings be upon him), while we too were sitting with him (peace and blessings be upon him). He said: O Messenger of Allah, as for sending salam upon you, we know it; but how should we send blessings (durud) upon you when we are praying? The Messenger of Allah (peace and blessings be upon him) fell silent, and remained silent for so long that we thought: if only this man had not asked him (peace and blessings be upon him). Then he (peace and blessings be upon him) said: "When you send blessings upon me, say: 'Allahumma salli 'ala Muhammadin an-Nabiyyi al-ummiyyi wa 'ala ali Muhammad, kama sallayta 'ala Ibrahima wa ali Ibrahim, wa barik 'ala Muhammadin an-Nabiyyi al-ummiyyi wa 'ala ali Muhammad, kama barakta 'ala Ibrahima wa ali Ibrahim, innaka Hamidun Majid' (O Allah, send blessings upon Muhammad (peace and blessings be upon him), the unlettered Prophet, and send blessings upon the family of Muhammad (peace and blessings be upon him), just as You sent mercy upon Ibrahim (peace be upon him) and the family of Ibrahim (peace be upon him); and send down blessings upon Muhammad, the unlettered Prophet (peace and blessings be upon him), and upon his (peace and blessings be upon him) family too, just as You sent down blessings upon Ibrahim (peace be upon him) and his family. Indeed, You are Praiseworthy, Glorious)."
Ali (al-Daraqutni) said: This chain is hasan (good) and connected.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3965