Musnad Ahmad 30:3317

Ali ibn 'Asim narrated to us, An-Nahhas ibn Qahm narrated to me, from Shaddad Abu 'Ammar, from Abu Hurairah, 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."