It wasnarrated from Thawban, the freed slave of the Messenger ofAllah(ﷺ), that the Messenger of Allah (ﷺ) said:
"Whoever fastssixdays after the Fitr will have completed the year, for whoever doesagood deed will have the reward of ten like it." (ID)
English reference (ID): Vol. 1, Book 7, Hadith 1715 (ID)
Arabic reference (ID): Book 7, Hadith 1786 (ID)