Musannaf Ibn Abi Shaybah

Hazrat Yahya bin Saeed says that I mentioned to Hazrat Qasim that a man from Yemen told me that when the Imam finishes the prayer with salam, people say "Allahu Akbar" or "La ilaha illallah" three times. Upon this, Hazrat Qasim said, "By God! Hazrat Ibn Zubair used to do the same."