Compiled by Imam Yahya ibn Sharaf al-Nawawi
عن أبي أيوب رضي الله عنه أن رسول الله صلى الله عليه وسلم قال: "من صام رمضان ثم أتبعه بست من شوال كان كصيام الدهر" ((رواه مسلم)).
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].