Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(108)
Chapter: To recite (the Qur'an) quietly in the Zuhr and 'Asr prayers
(108)
باب مَنْ خَافَتَ الْقِرَاءَةَ فِي الظُّهْرِ وَالْعَصْرِ
Sahih al-Bukhari 777

Narrated Abu Ma`mar:

We said to Khabbab "Did Allah's Messenger (ﷺ) 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."

حَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا جَرِيرٌ، عَنِ الأَعْمَشِ، عَنْ عُمَارَةَ بْنِ عُمَيْرٍ، عَنْ أَبِي مَعْمَرٍ، قُلْتُ لِخَبَّابٍ أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقْرَأُ فِي الظُّهْرِ وَالْعَصْرِ قَالَ نَعَمْ‏.‏ قُلْنَا مِنْ أَيْنَ عَلِمْتَ قَالَ بِاضْطِرَابِ لِحْيَتِهِ‏.‏
Reference : Sahih al-Bukhari 777
In-book reference : Book 10, Hadith 172
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 744
  (deprecated numbering scheme)