Musnad Ahmad 26:126

'Abd al-Samad narrated to us, 'Abd al-'Aziz ibn Muslim narrated to us, Abu Ishaq al-Hamdani narrated to us, from Abu al-Ahwas, from Ibn Mas'ud, that the Messenger of Allah (may Allah bless him and grant him peace) said: "When the remaining third of the night is left, Allah (Mighty and Majestic is He) descends to the lowest heaven. Then the gates of heaven are opened, then He spreads His hand and says: 'Is there any petitioner who will be given his request?' He continues in that state until the dawn breaks."