Ibn ‘Abbas said he used to know that God’s Messenger had finished his prayer when he heard the <i>takbir</i>. (Bukhari and Muslim.)

عَنِ ابْنِ عَبَّاسٍ رَضِيَ اللَّهُ عَنْهُمَا قَالَ: كُنْتُ أَعْرِفُ انْقِضَاءَ صَلَاةِ رَسُولِ اللَّهِ صَلَّى الله عَلَيْهِ وَسلم بِالتَّكْبِيرِ