Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 727
Narrated Abu Ma'mar:
I asked Khabbab whether the Prophet 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 that?" He said, "From the movement of his beard."