Sahih Bukhari
Sahih Bukhari Volume 001, Book 012, Hadith Number 744.
Hadith Section/Book: Sahih Bukhari Sahih Bukhari Book 12. Charecteristics Of Prayer
Narated By 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."