(the freed slave of Ibn ʿAbbas) Ibn ʿAbbas told me, "In the lifetime of the Prophet ﷺ it was the custom to celebrate Allah's praises aloud after the compulsory congregational prayers." Ibn ʿAbbas further said, "When I heard the Dhikr, I would learn that the compulsory congregational prayer had ended."
أَخْبَرَهُ أَنَّ رَفْعَ الصَّوْتِ بِالذِّكْرِ حِينَ يَنْصَرِفُ النَّاسُ مِنَ الْمَكْتُوبَةِ كَانَ عَلَى عَهْدِ النَّبِيِّ ﷺ وَقَالَ ابْنُ عَبَّاسٍ كُنْتُ أَعْلَمُ إِذَا انْصَرَفُوا بِذَلِكَ إِذَا سَمِعْتُهُ