Sahih al-Bukhari 419
Sahih al-Bukhari · Book 8: Prayers (Salat) · 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)
Narrated Anas bin Malik:
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 " .
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.)"