Sahih Bukhari Volume 001, Book 012, 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."