(24) باب الأَمْرِ بِتَحْسِينِ الصَّلاَةِ وَإِتْمَامِهَا وَالْخُشُوعِ فِيهَا
Anas b. Malik reported. The Messenger of Allah (ﷺ) said:
Perform bowing and prostration well. By Allah, I see you even if you are behind me, or he said: (I see you) behind my back when you bow or prostrate.
Reference: Sahih Muslim 425 a
In-book reference: Book 4, Hadith 121
USC-MSA web (English) reference(deprecated numbering scheme): Book 4, Hadith 855