سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 1117
The Book of The At-Tatbiq (Clasping One's Hands Together)
The command to prostrate properly
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 .
It was narrated from Anas that: The Messenger of Allah (ﷺ) said: "Bow and prostrate properly, for by Allah (SWT) I can see you from behind my back when you bow and prostrate."
Sunan an-Nasa'i, 1117Sahih (Darussalam)