مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 959
Prayer
Making Mention of God after the Prayer - Section 1
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.)
Mishkat al-Masabih, 959مُتَّفق عَلَيْهِ (الألباني)