Sahih Bukhari Volume 1, Book 12, Hadith Number 713

Narated By Abu Ma’mar : We asked Khabbab whether Allah’s Apostle used to recite (the Qur’an) in the Zuhr and the ‘Asr prayers. He replied in the affirmative. We said, “How did you come to know about it?” He said, “By the movement of his beard.”