Musannaf Ibn Abi Shayba 3:2011

Sahl ibn Yusuf narrated to us, from Humayd, from Bakr, who said: Someone who saw Ibn al-Zubayr in one of the walled gardens of Mecca informed me. He said: He prayed the Asr or the Dhuhr prayer, then he prostrated. A man said to him, "You have prayed five rak'ahs." He replied, "I recited a surah that contains a prostration."