Language:
كتاب الصلاة
8  
Prayers (Salat)
(40)
Chapter: Preaching of the Imam to the people regarding the proper offering of As-Salat (the prayers) and the mention of the Qiblah (Kabah at Makkah)
(40)
باب عِظَةِ الإِمَامِ النَّاسَ فِي إِتْمَامِ الصَّلاَةِ، وَذِكْرِ الْقِبْلَةِ
Sahih al-Bukhari 419

Narrated Anas bin Malik:

The Prophet (ﷺ) led us in a prayer and then got up on the pulpit and said, "In your prayer and bowing, I certainly see you from my back as I see you (while looking at you.)"

حَدَّثَنَا يَحْيَى بْنُ صَالِحٍ، قَالَ حَدَّثَنَا فُلَيْحُ بْنُ سُلَيْمَانَ، عَنْ هِلاَلِ بْنِ عَلِيٍّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ صَلَّى بِنَا النَّبِيُّ صلى الله عليه وسلم صَلاَةً ثُمَّ رَقِيَ الْمِنْبَرَ، فَقَالَ فِي الصَّلاَةِ وَفِي الرُّكُوعِ ‏ "‏ إِنِّي لأَرَاكُمْ مِنْ وَرَائِي كَمَا أَرَاكُمْ ‏"‏‏.‏
Reference : Sahih al-Bukhari 419
In-book reference : Book 8, Hadith 69
USC-MSA web (English) reference : Vol. 1, Book 8, Hadith 411
  (deprecated numbering scheme)