The Messenger of Allah ﷺ said, "Do you think my qiblah is only here in front of me? By Allah, neither your humility nor your bowing is hidden from me. I see you from behind my back."
أن رسول الله ﷺ قال: أترون قبلتي هاهنا؟ فوالله ما يخفى علي خشوعكم، ولا ركوعكم، إني لأراكم من وراء ظهري.
Malik related it in Qasr Safar (70) from Abu Zinad, from Araj, from Abu Hurairah, and he mentioned the hadith. And by way of Malik, Bukhari related it in Salah (418) and in Adhan (741), and Muslim in Salah (424), except that he added, "nor your sujud." Muslim also related it (423) by way of Saeed bin Abu Saeed Maqburi, from his father, from Abu Hurairah, who said, "The Messenger of Allah ﷺ once led us in prayer, then he turned and said, 'O so-and-so, will you not perfect your prayer? Does the one praying not consider, when he prays, how he prays? For he only prays for himself. By Allah, I can see behind me just as I see in front of me.'"The hadith also contains an encouragement toward humility, for completing the ruku and the sujud is among the greatest means of attaining it.