Sahih al-Bukhari 842
Sahih al-Bukhari · Book 10: Call to Prayers (Adhaan) · The Dhikr (remembering Allah by Glorifying, Praising and Magnifying Him) after As-Salat (the prayer)
Narrated Ibn `Abbas:
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ ، قَالَ حَدَّثَنَا سُفْيَانُ ، {عَنْ عَمْرٍو ،} قَالَ أَخْبَرَنِي أَبُو مَعْبَدٍ ، عَنِ ابْنِ عَبَّاسٍ ـ رضى الله عنهما ـ قَالَ كُنْتُ أَعْرِفُ انْقِضَاءَ صَلاَةِ النَّبِيِّ صلى الله عليه وسلم بِالتَّكْبِيرِ.
I used to recognize the completion of the prayer of the Prophet (ﷺ) by hearing Takbir.