[Al-Bhukari, Muslim 3/1685, At Trimidi 2/1038 , Ahmed 5/218]
Allah is the greatest
Ibn Abbas (R.A) said, we used to know that Allah's messenger had finished his prayer when we heard “Allahu Akbar”.
[Sahih Muslim vol. 1, 1209/1210, Bukhari Vol.1 Book 12/802]