Musnad Ahmad 30:2588

Waki’ narrated to us, he said: An-Nahhas ibn Qahm narrated to us, from Shaddad Abu ‘Ammar, from Abu Hurairah, he 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."