Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 713
Narrated 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."