Musnad Ahmad 299:13

Yazid ibn Harun narrated to us, Hammad ibn Salamah narrated to us, from Ali ibn Zayd, from al-Hasan, from Uthman ibn Abi al-As, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A caller calls out every night: 'Is there anyone supplicating so that he may be answered? Is there anyone asking so that he may be given? Is there anyone seeking forgiveness so that he may be forgiven?' until the dawn breaks."