The Messenger of Allah ﷺ said, "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as if he has fasted the entire year."
أن رسول الله ﷺ قال: من صام رمضان وأتبعه ستا من شوال، كان كصيام الدهر.
Muslim narrated it in the Book of Fasting (1164) through various routes from Saad bin Saeed, the brother of Yahya bin Saeed, from Umar bin Thabit bin Harith Khazraji Ansari, from Abu Ayub Ansari, who said, and he related it.Saad bin Saeed is Ibn Qays bin Amr Ansari, the brother of Yahya bin Saeed Ansari. Ahmad, Nasai, and others declared him daif on account of his poor memory. However, he was not alone in narrating this hadith, since Safwan bin Salim also related it along with him. Abu Dawud narrated it (2433), and Ibn Khuzayma graded it sahih (2114), as did Ibn Hibban (3634), and some of them narrated it with his narration joined to it.Safwan bin Salim is the Madinan, and he is trustworthy. Abu Hatim and others confirmed his reliability, and his narration strengthens the hadith of Saad bin Saeed, indeed it is even more deserving of reliance than his.Tirmidhi said (759), after citing the hadith, "Some scholars of hadith have criticized Saad bin Saeed regarding his memory."I say, this weakness has been removed by the corroboration of Safwan bin Salim. Indeed, Tahawi said in his Mushkil (6/119-121), "This hadith was one that did not sit strongly in our hearts, given what was said about Saad bin Saeed in narration among the scholars of hadith and their reluctance regarding him, until we found that those we mentioned, who were people of stature and reliability in narration and steadfastness therein, had also taken it from him. So we cited his hadith for that reason." He then presented the chains of Safwan bin Salim, Zayd bin Aslam, Yahya bin Saeed Ansari, and Abd Rabbih bin Saeed Ansari, all of them narrating from Umar bin Thabit.However, Umar bin Thabit, who is Ibn Harith Mazini Khazraji Ansari Madani, was the sole narrator of this particular report, but his being the sole narrator does not weaken it, for he is among the trustworthy scholars of Madinah, and his hadith is further supported by the hadith of Thawban and others.