Sahih Bukhari Volume 008, Book 075, Hadith Number 340.

Narated By '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-lillahi,' 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."