Muwatta Imam Malik Book of Shortening the Prayer, Hadith 73

Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah ﷺ said, "Do you see the direction I am facing here? By Allah, neither your concentration nor your ruku is hidden from me. I can see you behind my back."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ أَبِي الزِّنَادِ، عَنِ الأَعْرَجِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ " أَتَرَوْنَ قِبْلَتِي هَاهُنَا فَوَاللَّهِ مَا يَخْفَى عَلَىَّ خُشُوعُكُمْ وَلاَ رُكُوعُكُمْ إِنِّي لأَرَاكُمْ مِنْ وَرَاءِ ظَهْرِي " .