Abu Ayyub (May Allah be pleased with him) reported:
The Messenger of Allah (ﷺ) said, "He who observes As-Saum (the fasts) in the month of Ramadan, and also observes As-Saum for six days in the month of Shawwal, it is as if he has observed As-Saum for the whole year."[Muslim]. (DE)
App reference (DE): Book 9, Hadith 264 (DE)
Arabic/English book reference (DE): Book 9, Hadith 1254 (DE)