I heard the Prophet ﷺ saying, "Perform the bowing and the prostration properly (with peace of mind), for, by Him in Whose Hand my soul is, I see you from behind my back when you bow and when you prostrate."
أَنَّهُ سَمِعَ النَّبِيَّ ﷺ يَقُولُ أَتِمُّوا الرُّكُوعَ وَالسُّجُودَ فَوَالَّذِي نَفْسِي بِيَدِهِ إِنِّي لأَرَاكُمْ مِنْ بَعْدِ ظَهْرِي إِذَا مَا رَكَعْتُمْ وَإِذَا مَا سَجَدْتُمْ