We used to know that Messenger of Allah ﷺ had finished his prayer when we heard the takbir (Allah-O-Akbar).  

أَخْبَرَنِي بِذَا أَبُو مَعْبَدٍ ثُمَّ أَنْكَرَهُ بَعْدُ عَنِ ابْنِ عَبَّاسٍ قَالَ كُنَّا نَعْرِفُ انْقِضَاءَ صَلاَةِ رَسُولِ اللَّهِ ﷺ بِالتَّكْبِيرِ  

We knew the finishing of the prayer of the Messenger of Allah ﷺ 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.  

مَا كُنَّا نَعْرِفُ انْقِضَاءَ صَلاَةِ رَسُولِ اللَّهِ ﷺ إِلاَّ بِالتَّكْبِيرِ قَالَ عَمْرٌو فَذَكَرْتُ ذَلِكَ لأَبِي مَعْبَدٍ فَأَنْكَرَهُ وَقَالَ لَمْ أُحَدِّثْكَ بِهَذَا قَالَ عَمْرٌو وَقَدْ أَخْبَرَنِيهِ قَبْلَ ذَلِكَ  

Dhikr (mentioning the name of Allah) in a loud voice after obligatory prayers was (a common practice) during the lifetime of the Messenger of Allah ﷺ ; and when I heard that I came to knew that they (the people) had finished the prayer.  

أَخْبَرَهُ أَنَّ رَفْعَ الصَّوْتِ بِالذِّكْرِ حِينَ يَنْصَرِفُ النَّاسُ مِنَ الْمَكْتُوبَةِ كَانَ عَلَى عَهْدِ النَّبِيِّ ﷺ وَأَنَّهُ قَالَ قَالَ ابْنُ عَبَّاسٍ كُنْتُ أَعْلَمُ إِذَا انْصَرَفُوا بِذَلِكَ إِذَا سَمِعْتُهُ