Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 744
Narrated Abu Ma'mar:
We said to Khabbab "Did Allah's Apostle used to recite in Zuhr and '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."