Musnad Ahmad 26:718

I read to my father: Mu'awiyah ibn 'Amr narrated to us, he said: Za'idah narrated to us, Ibrahim al-Hajari narrated to us, from Abu al-Ahwas, from 'Abdullah, from the Prophet (peace and blessings of Allah be upon him), who said: "Indeed, Allah, the Almighty and Majestic, opens the gates of the heaven in the remaining third of the night, then He descends to the lowest heaven, then He spreads out His hand, then He says: 'Is there any servant who asks of Me, so that I may give to him?' until the dawn breaks."