Sahih Bukhari
Characteristics of Prayer
Volume: 001 / Book Number: 012 / Hadith Number: 728
Narrated Abu Ma'mar:
I asked Khabbab bin Al-Art whether the Prophet used to recite the Qur'an in the Zuhr and the 'Asr prayers. He replied in the affirmative. I said, "How did you come to know that?" He replied, "From the movement of his beard."