Musnad Ahmad 26:369

Abu Sa’id narrated to us, Zaidah narrated to us, Mansur narrated to us, from Shaqiq, from ‘Abdullah, who said: When we used to pray behind the Messenger of Allah (peace and blessings of Allah be upon him), one of us would say in his prayer: "Peace be upon Allah, peace be upon so-and-so," specifying [individuals]. One day, the Messenger of Allah (peace and blessings of Allah be upon him) said to us: "Indeed, Allah the Almighty is As-Salam (the Peace). So when one of you sits in his prayer, let him say: 'At-tahiyyatu lillahi, was-salawatu wat-tayyibat. As-salamu ‘alaika ayyuhan-nabiyyu wa rahmatullahi wa barakatuh. As-salamu ‘alaina wa ‘ala ‘ibadillahis-salihin'—for if you say that, you have sent peace upon every servant in the heavens and the earth—'Ash-hadu an la ilaha illallah, wa ash-hadu anna Muhammadan ‘abduhu wa rasuluh.' Then let him choose whatever supplication he wishes—or whatever he likes."