Bukhari

Narrated `Abdullah bin `Amr: Abu Bakr As-Siddiq said to the Prophet, "Teach me an invocation with which I may invoke (Allah) in my prayer." The Prophet said, "Say: Allahumma inni zalamtu nafsi zulman kathiran wala yaghfirudhdhunuba illa anta, Faghfirli maghfiratan min indika war-hamni, innaka antalGhafur-Rahim."

Narrated `Aisha: The Verse: 'Neither say your prayer aloud, nor say it in a low tone.' (17.110) was revealed as regards invocation.

Narrated `Abdullah: We used to say in the prayer: 'AsSalam be on Allah, As-Salam be on so-and so.' So one day the Prophet said to us, "Allah Himself is As-Salam; when anyone of you sits during his prayer, he should say: 'at-tah, iyyatu-li l-lahi,' up to 'As-Salihin,' (All the compliments are for Allah ...righteous people) for when he recites this, then he says his Salam to all the righteous people present in the heavens and on the earth. Then he should say, 'I testify that none has the right to be worshipped except Allah, and that Muhammad is His slave and His Apostle,' and then he can select whatever he likes to celebrate (Allah's) Praises."