ENOfficial
Ibn ‘Abbas said he used to know that God’s Messenger had finished his prayer when he heard the takbir. (Bukhari and Muslim.)
Source: sunnah.com
Other translations
Arabic
عَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: كُنْتُ أَعْرِفُ انْقِضَاءَ صَلَاةِ رَسُولِ اللَّهِ صَلَّى الله عَلَيْهِ وَسلم بِالتَّكْبِيرِ
sunnah.com