Musannaf Ibn Abi Shayba 3:224

Abu Bakr narrated to us, he said: Jarir narrated to us, from Abd al-Aziz ibn Rufay’, from a man from the people of Medina, from the Prophet (may Allah bless him and grant him peace), that he heard the sound of sandals while he was prostrating. When he finished his prayer, he said: "Who is this whose sandals I heard?" He said: "I am, O Messenger of Allah." He said: "What did you do?" He said: "I found you prostrating, so I prostrated." He said: "Do thus, and do not count it [as a rak'ah]. Whoever finds me bowing, standing, or prostrating, let him be with me in the state that I am in."