The Messenger of Allah ﷺ said, "Whoever fasts Ramadan and follows it with six days of Shawwal, it is as though he has fasted the entire year."
قال رسول الله ﷺ: من صام رمضان وأتبعه بست من شوال فكأنما صام الدهر.
Bazzar narrated it in Kashf Astar (1061) from Muhammad bin Miskin, who said, Amr narrated to us, who said, Suhayl narrated to us, from his father, from Abu Hurairah, and he related it. Its chain of transmission is hasan because of Amr, who is Ibn Abu Salama Tanisi Dimashqi, one of the narrators accepted by the entire community, except that he errs slightly, though perhaps he did not err in this particular hadith. Bazzar also narrated it (1060) with another chain of transmission and found fault with it. That is why Haythami said in Majma al-Zawaid (3/183), "Bazzar narrated it, and it has multiple paths, the narrators of some of which are the narrators of the sahih collections."On this subject there is also a report from Jabir bin Abdullah, who said, the Messenger of Allah ﷺ said, "Whoever fasts Ramadan and then six days of Shawwal, it will be as if he fasted the whole year." Imam Ahmad narrated it (14302), as did Bazzar in Kashf Astar (1062), Tahawi in his Mushkil (2350), Bayhaqi (4/293), and Abd bin Humayd (1116), all of them through the path of Amr bin Jabir Hadrami Abu Zura, who heard Jabir bin Abdullah say (and he related the hadith). Bazzar said, "Amr alone transmitted it." Haythami said in Majma al-Zawaid (3/183), after attributing it to Ahmad, Bazzar, and Tabarani in Awsat, "It contains Amr bin Jabir, who is daif."As for what Tabarani narrated in Awsat (7603) from Abu Hurairah as a raised report, which contains, "Whoever fasts six days in succession after the Fitr, it is as if he fasted the whole year," the word "in succession" contains an element of irregularity. Haythami said in Majma al-Zawaid (3/183–184), "Tabarani narrated it in Awsat, and it contains someone I do not recognize."The majority of scholars among the hadith specialists and jurists took up these hadiths and held that fasting six days of Shawwal is recommended. Some preferred that it be from the beginning of the month, though if one fasted them scattered throughout Shawwal before it ended, that would also be permissible.Malik disliked that these days be attached directly to Ramadan. He said, "No report of this has reached me from any of the early generations, and scholars dislike this and fear it introduces an innovation, and fear that people of ignorance and coarseness might attach to Ramadan what does not belong to it, even if they saw that scholars permitted it and saw them practicing it." End of quote.Ibn Abdul-Barr said, as quoted from him by Ibn Qayyim in Tahzib Sunan, "The hadith of Abu Ayub did not reach Malik, even though it is a Madinan hadith, and it is impossible to encompass the knowledge held by specialists individually. What Malik disliked, he explained and clarified, out of fear that it might be added to the obligation of Ramadan and that this might spread among the common people. He was cautious and extremely careful regarding matters of religion. As for fasting the six days seeking the reward and following the interpretation brought by Thawban, Malik would not dislike that, Allah willing, because fasting is a shield and its virtue is well known, one who abandons his food and drink for the sake of Allah performs an act of righteousness and goodness, and Allah Most High has said, "And do good, that you may succeed" [Hajj 22:77]. Malik was not ignorant of any of this. He disliked it only out of fear for people of ignorance and coarseness, should this practice continue, and out of concern that it might come to be counted among the obligatory acts of fasting, added on to Ramadan."See also Istidhkar (10/259). Ibn Abdul-Barr concluded by saying, "It may be that he was unaware of the hadith, for had he known it, he would have held to it, and Allah knows best."I say, Shafii, may Allah have mercy on him, spoke truly when he said, "There is no sahih hadith that has not been preserved, not by a single individual, but collectively among the various members of the nation." Imam Malik, the imam of the city of the Hijrah, was unaware of the hadith of Abu Ayub concerning the virtue of fasting six days of Shawwal, even though it is a Madinan hadith, so how much more so with others?