Are the statements of the scholar who indicated that there are no authentic hadiths proving the Prophet, may God's prayers and peace be upon him, fasted the six days of Shawwal, and that the hadiths reported in this regard from Sa'd ibn Sa'id are weak and impugned, correct?

Sa'd ibn Sa'id ibn Qays, the narrator of the hadith "Whoever fasts Ramadan then follows it with six days of Shawwal...", was deemed weak by some scholars due to his poor memory, while others considered him trustworthy. Imam Muslim narrated his hadith, and Imam Bukhari cited him as a witness, which indicates their acceptance of him. Ibn Hibban and Ibn 'Adi supported him, stating that his weakness was not severe. The hadith is strong due to contextual evidence and corroborating narrations; many trustworthy narrators reported it from him, and it has follow-ups and corroborating narrations from the hadith of Safwan ibn Sulaym, Yahya ibn Sa'id, Thawban, and others, which elevate the hadith to the level of authenticity.

There is no evidence to negate the Prophet's (peace be upon him) fasting of the six days of Shawwal. The absence of its narration does not mean it did not occur, as he might have fasted without people knowing, or his wives might not have conveyed his fasting. Moreover, the existence of a prophetic directive for an action is sufficient for its recommendation, even if the Prophet's (peace be upon him) personal practice of it is not known. Most scholars considered fasting these days recommendable, including Imam Ahmad, who did not deem the narration void despite his weakening of Sa'd, and he held the view that fasting them is recommendable.