Sahih Al Bukhari Hadith Number 746

Narrated Abu Ma`mar: We asked Khabbab whether Allah’s Messenger (peace be upon him) 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.”

10  Call to Prayers (Adhaan)
Hadith number 746
Volume no 1
Chapter: 91 (To cast a look at the Imam during As-Salat (the prayer)