Musnad Ahmad 30:3349

Ali ibn Asim narrated to us, al-Nahhas ibn Qahm informed us, from Abu Ammar Shaddad, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever maintains the Duha prayer, his sins will be forgiven, even if they were like the foam of the sea."