Narrated Abu Huraira:
Allah’s Apostle said, “Do you consider or see that my face is towards the Qibla? By Allah, neither your submissiveness nor your bowing is hidden from me, surely I see you from my back.”
Narrated Anas bin Malik:
The Prophet led us in a prayer and then got up on the pulpit and said, “In your prayer and bowing, I certainly see you from my back as I see you (while looking at you.)”
Weird, right? Scary too, especially for those offering Salah behind him. It was one of the miracles of the Prophet (SAW).