The Messenger of Allah ﷺ said, "The closest a servant is to his Lord is when he is prostrating, so make abundant supplication."
حدثنا هارون قال عبد الله وسمعته أنا من هارون قال حدثنا ابن وهب عن عمرو عن عمارة بن غزية عن سمي مولى أبي بكر أنه سمع أبا صالح ذكوان يحدث
أن رسول الله ﷺ قال أقرب ما يكون العبد من ربه وهو ساجد فأكثروا الدعاء
Its isnad is sahih. Muslim narrated it, 1/350, no. 482, in the Book of Prayer, chapter on what is said during ruku, and Abu Dawud, 1/231, no. 875, in the Book of Prayer, chapter on the supplication during ruku and sujud, with the same wording, and Nasai, 2/226, no. 1137, in the Book of Opening the Prayer, chapter on the closest a servant comes, with similar wording.