It wasnarrated from Abu Dharr that the Messenger of Allah (ﷺ)said:
"Whoever fasts three days in every month, that is fasting for alifetime." Then, in testimony of that, Allah revealed: "Whoeverbringsa good deed shall have ten time the like thereof to hiscredit."[6:160] So one day is equivalent to ten (in reward).
English reference: Vol. 1, Book 7, Hadith 1708
Arabic reference: Book 7, Hadith 1779