It is narrated from Sayyiduna Jabir (may Allah be pleased with him) that I heard the Prophet (peace and blessings be upon him) say: Whoever fasts the month of Ramadan and then follows it with six days of fasting in Shawwal, it is as if he has fasted for the whole year.
Hadith Referenceمسند احمد / مسند المكثرين من الصحابة / 14302
Hadith Gradingحکم دارالسلام:صحيح لغيره، وهذا إسناد ضعيف لضعف عمرو بن جابر الحضرمي، وانظر: 23533