Sayyiduna Abu Humayd al-Sa'idi (may Allah be pleased with him) says: We said: O Messenger of Allah, how should we send blessings upon you? The Messenger of Allah (peace and blessings be upon him) said: "Say: 'Allahumma salli 'ala Muhammadin wa azwajihi wa dhurriyyatih, kama sallayta 'ala ali Ibrahim, wa barik 'ala Muhammadin wa azwajihi wa dhurriyyatih, kama barakta 'ala ali Ibrahim, innaka Hamidun Majid' (O Allah, send Your mercy upon Muhammad (peace and blessings be upon him), upon his pure wives and upon his offspring, just as You sent mercy upon the family of Ibrahim (peace be upon him); and send down blessing upon Muhammad (peace and blessings be upon him), upon his wives and upon his offspring, just as You sent down blessing upon the family of Ibrahim (peace be upon him). Indeed, You are Praiseworthy, Glorious)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2863