Musnad Ahmad 3894
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: When we prayed behind the Messenger of Allah (ﷺ), one of us would say in his prayer: Peace be upon Allah, peace be upon So and so, mentioning some specific names. Then the Messenger of Allah (ﷺ) said to us one day: “Allah, may He be glorified and exalted, is as-Salam, so when one of you sits in his prayer, let him say: "All compliments, prayers and pure words are due to Allah. Peace be upon you, O Prophet, and the mercy of Allah and His blessings. Peace be upon us, and on the righteous slaves of Allah’ - If you say that, you will have sent salams upon every slave (of Allah) in the heavens and on earth - ‘I bear witness that there is no god but Allah, and I bear witness that Muhammad is His slave and Messenger.’ Then let him choose whatever supplication he wants - or he likes.”