Musnad Ahmad 26:514

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Shaqiq ibn Salamah, from Abdullah ibn Mas'ud, who said: When we sat with the Prophet, peace and blessings of Allah be upon him, in the prayer, we used to say: "Peace be upon Allah before His servants, peace be upon Jibril, peace be upon Mika'il, peace be upon so-and-so, peace be upon so-and-so." He said: The Messenger of Allah, peace and blessings of Allah be upon him, heard us and said: "Indeed, Allah is As-Salam (The Peace). So when one of you sits in the prayer, let him say: 'At-tahiyyatu lillahi, was-salawatu wat-tayyibat. As-salamu 'alayka ayyuhan-nabiyyu wa rahmatullahi wa barakatuh. As-salamu 'alayna wa 'ala 'ibadillahis-salihin' — for if he says it, it reaches every righteous servant in the heaven and the earth — 'Ash-hadu an la ilaha illallah, wa ash-hadu anna Muhammadan 'abduhu wa rasuluh.' Then let him choose from the supplications whatever he wishes."