Sahih Muslim Book 4, Hadith Number 1211.

Chapter : Taslim at the completion of the prayer.

Ibn ‘Abbas reported: Dhikr (mentioning the name of Allah) in a loud voice after obligatory prayers was (a common practice) during the lifetime of the Apostle of Allah (may peace be upon him); and when I heard that I came to knew that they (the people) had finished the prayer.