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 Book of Prayer (482) via Sumayy, the freed slave of Abu Bakr, that he heard Abu Salih Zakwan narrating from Abu Hurairah, and he related the hadith.