Sahih Al Bukhari Hadith Number 777

Narrated Abu Ma`mar: We said to Khabbab “Did Allah’s Messenger (peace be upon him) 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.”

10  Call to Prayers (Adhaan)
Hadith number 777
Volume no 1
Chapter: 108 (To recite (the Qur’an) quietly in the Zuhr and ‘Asr prayers)