The Messenger of Allah ﷺ said, "The closest a servant comes to his Lord is when he is in prostration, so make abundant supplication."
أن رسول الله ﷺ قال: أقرب ما يكون العبد من ربه وهو ساجد فأكثروا الدعاء.
Muslim narrated it in the chapter on prayer (482), through the route of Sumayy, the freed slave of Abu Bakr, that he heard Abu Salih Zakwan narrating from Abu Hurairah, and he mentioned it.