Mishkat al-Masabih 4 Prayer كتاب الصلاة 292

عَنْ أَنَسٍ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «أَقِيمُوا الرُّكُوعَ وَالسُّجُودَ فَوَاللَّهِ إِنِّي لأَرَاكُمْ من بعدِي»

Anas reported God’s Messenger as saying, “Perform the bowing and the prostration properly, for I swear by God that I can see you behind me.” (Bukhari and Muslim.)