The Messenger of Allah ﷺ said, "Allah descends to the lowest heaven every night when the first third of the night has passed, and He says, 'I am the Sovereign, I am the Sovereign. Who is calling upon Me, that I may answer him? Who is asking of Me, that I may grant him? Who is seeking My forgiveness, that I may forgive him?' And He continues in this way until the light of Fajr breaks."
وأن رسول الله ﷺ قال ينزل الله عز وجل إلى السماء الدنيا كل ليلة حين يمضي ثلث الليل الأول فيقول أنا الملك مرتين من ذا الذي يدعوني فأستجيب من الذي يسألني فأعطيه من ذا الذي يستغفرني فأغفر له فلا يزال كذلك حتى يضيء الفجر
Its isnad is sahih. Muslim narrated it, 1/522, no. 758, in the Book of the Prayer of Travelers, chapter on the encouragement to supplicate and remember Allah in the last part of the night, and Tirmidhi, 2/307, no. 446, and he said, hasan sahih.