The Messenger of Allah ﷺ said, "Complete the bowing and prostration properly, for by Allah, I see you from behind me," and sometimes he said, "from beyond my back, when you bow and when you prostrate."
حدثنا يحيى عن شعبة حدثنا قتادة
قال رسول الله ﷺ أتموا الركوع والسجود فوالله إني لأراكم من بعدي وربما قال من وراء ظهري إذا ركعتم وإذا سجدتم
Its isnad is sahih, and it is found in Bukhari 2/225, no. 742, Fath, with the wording "Establish," in the chapter on adhan, and on humility in prayer, and in Muslim with the same wording, 1/320, no. 425, in the chapter on prayer, on the command to perfect the prayer.