Language:
كتاب الأذان
10  
Call to Prayers (Adhaan)
(91)
Chapter: To cast a look at the Imam during As-Salat (the prayer)
(91)
باب رَفْعِ الْبَصَرِ إِلَى الإِمَامِ فِي الصَّلاَةِ
Sahih al-Bukhari 746

Narrated Abu Ma`mar:

We asked Khabbab whether Allah's Messenger (ﷺ) 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."

حَدَّثَنَا مُوسَى، قَالَ حَدَّثَنَا عَبْدُ الْوَاحِدِ، قَالَ حَدَّثَنَا الأَعْمَشُ، عَنْ عُمَارَةَ بْنِ عُمَيْرٍ، عَنْ أَبِي مَعْمَرٍ، قَالَ قُلْنَا لِخَبَّابٍ أَكَانَ رَسُولُ اللَّهِ صلى الله عليه وسلم يَقْرَأُ فِي الظُّهْرِ وَالْعَصْرِ قَالَ نَعَمْ‏.‏ قُلْنَا بِمَ كُنْتُمْ تَعْرِفُونَ ذَاكَ قَالَ بِاضْطِرَابِ لِحْيَتِهِ‏.‏
Reference : Sahih al-Bukhari 746
In-book reference : Book 10, Hadith 140
USC-MSA web (English) reference : Vol. 1, Book 12, Hadith 713
  (deprecated numbering scheme)