Ibn 'Abbas said: We used to know that Allaah's Messenger (sallAllaahu alayhi wa sallam) had finished his prayer when we heard the takbir (Allaah-O-Akbar).
Ibn 'Abbas reported: We knew the finishing of the prayer of the Messenger of Allaah (sallAllaahu alayhi wa sallam) through takbir. 'Amr (b. Dinar) said: I made a mention of it to Abu Mas'ud. hue he rejected it and said: I never narrated it to you. 'Amr said: He did narrate it before this.
Ibn 'Abbas reported: Dhikr (mentioning the name of Allaah) in a loud voice after obligatory prayers was (a common practice) during the lifetime of the Prophet of Allaah (sallAllaahu alayhi wa sallam) ; and when I heard that I came to knew that they (the people) had finished the prayer.