Sahl bin Mu'adh bin Anas al-Juhani (may Allah have mercy on him) narrates from his father that the Messenger of Allah (peace and blessings be upon him) said: "Whoever remains seated in his place after finishing the Fajr prayer and then prays the two rak'ahs of the Duha prayer, saying nothing but good in the meantime, all his sins will be forgiven, even if they are more than the foam of the sea."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 4907