It is from the sunnah to say this dua during sujood/prostration,
ุงูููู ุงุบูุฑ ูู ุฐูุจู ููู ุฏูู ูุฌููุ ูุฃููู ูุขุฎุฑูุ ูุนูุงููุชู ูุณุฑู
O Allah! Forgive all my sins, the small and the great, first and the last, the open and the secret