The Prophet (peace and blessings of Allah be upon him) explained this when he said: “Whoever fasts for six days after (‘Eid) al-Fitr has completed the year: (whoever does a good deed (hasanah) will have ten hasanah like it).” According to another report: “Allah has made for each hasanah ten like it, so a month is like fasting ten months, and fasting six days completes the year.” (al-Nasai and Ibn Majah. See also Sahih al-Targhib wa’l-Tarhib, 1/421)