Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1118 (DarusSalam) ⓘ
It was narrated from Anas that: The Messenger of Allah said: "Bow and prostrate properly, for by Allah (SWT) I can see you from behind my back when you bow and prostrate."
أَخْبَرَنَا إِسْحَاقُ بْنُ إِبْرَاهِيمَ، قَالَ أَنْبَأَنَا عَبْدَةُ، عَنْ سَعِيدٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم قَالَ " أَتِمُّوا الرُّكُوعَ وَالسُّجُودَ فَوَاللَّهِ إِنِّي لأَرَاكُمْ مِنْ خَلْفِ ظَهْرِي فِي رُكُوعِكُمْ وَسُجُودِكُمْ " .
also referenced under: Hadith 1117 (Reference) & Book 12, Hadith 89 (In-Book)