The Prophet (Sall’Allahu alaihi Wasallam ) 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.”

📚[Sunan al-Nisaa’i & Sunan Ibn Maajah. See also Saheeh al-Targheeb wa’l-Tarheeb, 1/421]