Mishkat al-Masabih 4 Prayer كتاب الصلاة 382

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

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