Muwatta Malik 401

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

Yahya related to me from Malik from Abu'z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, 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

Reference

Muwatta Malik 401

In-book

Book 9, Hadith 76