Sahih al-Bukhari, Hadith 419 ⓘ
Narrated Anas bin Malik: The Prophet 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 " .
also referenced under: Book 8, Hadith 69 (In-Book) & Volume 1, Book 8, Hadith 411 (USC-MSA)