Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.139B.
Abu Mas’ud Al-Ansari reported the following account from Bashir ibn Sa’d: “I asked Prophet, peace be upon him, ‘O Messenger of Allah, Allah has commanded us to invoke blessings upon you. How should we do it?’ The Messenger of Allah, peace be upon him, remained silent until we wished we had not asked him. Then he told us to say,
‘Allahumma salli ‘ala muhammadin wa ‘ala ali muhammadin kama sallayta ‘ala ali ibrahima wa barik ‘ala muhammadin wa ‘ala ali muhammadin kama barakta ‘ala ibrahima wa ‘ala ali ibrahima fil ‘alamin, innaka hammidum majeed (O Allah, bless Muhammad and the family of Muhammad as You blessed the family of Ibrahim, and give baraka to Muhammad and the family of Muhammad as You gave baraka to the family of Ibrahim, in all the worlds. You are worthy of Praise and Glorious), and then he told us to give the taslim as you have learned it.” (Muslim)
Abdullah ibn Mas’ud said, “When you invoke blessings upon the Prophet do it in the best manner, because you don’t know whether or not your blessings may be presented to him. The Companions asked him to teach them how to do it. He replied, ‘Say, “Allahumma ‘ij’al salawataka wa rahmataka wa barakataka ‘ala sayyidil mursalin, wa imamal mutaqadimin wa khatimin nahiyyin muhammadin ‘abduka wa rasuluka imamal khair wa qaidil khair wa rasulir rahmati. Allahumma b’athu maqaman yaghbatuhu bihil awalun. Allahumma salli ‘ala muhammadin wa ‘ala ali muhammadin kama sallaita ‘ala ibrahima wa ali ibrahima innaka hamidum majeed. Allahumma barik ‘ala muhammadin wa ‘ala ali muhammadin kama barakta ‘ala ibrahima wa ‘ala ali ibrahima innak hamidum majeed (O Allah, shower Your blessings, mercy, and favors on the Chief of the Messengers, and the Chief of the Foremost, the Seal of the Prophets, Muhammad, Your servant and Your Messenger, the leader of the virtuous, the chief of the virtuous, the Messenger of Mercy, grant him a station that is the happy goal of the most excellent, O Allah, bless Muhammad and his family as You blessed Ibrahim and the family of Ibrahim. You are indeed Praiseworthy and Glorious. O Allah, give baraka to Muhammad and his family as you gave baraka to Ibrahim and his family, You are indeed Praiseworthy and Glorious”‘.” (Ibn Majah)