It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "When one of you recites the tashahhud, after it he should say: 'Allahumma salli 'ala Muhammadin wa 'ala ali Muhammad, wa barik 'ala Muhammadin wa 'ala ali Muhammad, warham Muhammadan wa ala Muhammad, kama sallayta wa barakta wa tarahhamta 'ala Ibrahima wa 'ala ali Ibrahim, innaka Hamidun Majid' (O Allah, send blessings upon Muhammad (peace and blessings be upon him) and upon his family, send down blessings upon Muhammad (peace and blessings be upon him) and upon his family, and have mercy on Muhammad (peace and blessings be upon him) and the family of Muhammad (peace and blessings be upon him), just as You sent down mercies and blessings upon Ibrahim (peace be upon him) and the family of Ibrahim (peace be upon him) and showed them mercy. Indeed, You are Praiseworthy, Glorious)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3966