(Imām Bukhārī narrates): Ishāq ibn Nasr narrated to us, he said: 'Abdur-Razzāq narrated to us, he said: Ibn Juraij informed us, he said: 'Amr informed me that Abū Ma'bad, the freed slave of Ibn 'Abbās, informed him that Ibn 'Abbās, may Allāh be pleased with them both, informed him that:
in the time of the Messenger of Allāh ﷺ, it was the practice of the people that they would remember Allāh the Exalted after finishing the prayer. Ibn 'Abbās said: When I heard this remembrance, I would know that the people had finished the prayer.
حَدَّثَنَا إِسْحَاقُ بْنُ نَصْرٍ، قَالَ حَدَّثَنَا عَبْدُ الرَّزَّاقِ، قَالَ أَخْبَرَنَا ابْنُ جُرَيْجٍ، قَالَ أَخْبَرَنِي عَمْرٌو، أَنَّ أَبَا مَعْبَدٍ، مَوْلَى ابْنِ عَبَّاسٍ أَخْبَرَهُ أَنَّ ابْنَ عَبَّاسٍ ـ رضى الله عنهما ـ أَخْبَرَهُ أَنَّ رَفْعَ الصَّوْتِ بِالذِّكْرِ حِينَ يَنْصَرِفُ النَّاسُ مِنَ الْمَكْتُوبَةِ كَانَ عَلَى عَهْدِ النَّبِيِّ صلى الله عليه وسلم. وَقَالَ ابْنُ عَبَّاسٍ كُنْتُ أَعْلَمُ إِذَا انْصَرَفُوا بِذَلِكَ إِذَا سَمِعْتُهُ.
Reference: Sahih Al Bukhari 841