The Prophet ﷺ said, "Do not fast on Saturday except for what has been made obligatory upon you. If one of you finds nothing but the bark of a grapevine or the twig of a tree, let him chew it."
أن النبي ﷺ قال: لا تصوموا يوم السبت إلا فيما افترض عليكم، وإن لم يجد أحدكم إلا لحاء عنبة، أو عود شجرة فليمضغه.
Abu Dawud (2421), Tirmidhi (744), Ibn Majah (1726), and Ahmad (27075) narrated it. Ibn Khuzayma (2164) and Hakim (1/435) graded it sahih, all of them from the hadith of Thawr, from Khalid bin Madan, from Abdullah bin Busr, from his sister Samma, and he narrated it. Its chain of transmission is sahih.Imam Ahmad narrated it with another chain (17686) from Abdullah bin Busr, who is also a companion, and "his sister" is not mentioned in this chain. Abu Dawud said, "This hadith is abrogated." He also said, "Abdullah bin Busr is from Hims, and this hadith is abrogated, abrogated by the hadith of Juwayria." Abu Dawud reported in his Sunan that Malik said, "This hadith is false." Bayhaqi (4/302–303) reported that Uzai said, "I kept it hidden for a long time, then I saw it spread widely." It has been said that there is inconsistency in it, as Nasai stated.In contrast, Tirmidhi said, "It is a hasan hadith. The meaning of its disapproval here is that a man singles out Saturday for fasting, because the Jews venerate Saturday." Hakim said, "It is a hadith that is sahih according to the criterion of Bukhari." He also said, it has a counterpart with a sahih chain, and he cited it. So he mentioned the hadith of Juwayria bint Harith, as previously mentioned in the chapter on the prohibition of fasting Friday alone.Nawawi rejected Malik's statement, "This hadith is false," in Sharh Muhazzab (6/439), saying, "This statement is not acceptable, for the imams have graded it sahih." He also responded to Abu Dawud's claim that it is abrogated, saying, "It is not as he said."Once you know this, know that this hadith has a sahih chain, and its narrators are trustworthy. Thawr is Ibn Yazid, the Himsi, trustworthy and reliable. Khalid bin Madan, the Kalai, is also from Hims, trustworthy, devout, and one of the narrators relied upon by the major hadith compilers. Abdullah bin Busr is a minor companion, and he was the last of the companions to die in Syria.Thawr bin Yazid was corroborated, in what Ahmad narrated (27077) from Hakam bin Nafi, who said, Ismail bin Ayyash narrated to us, from Muhammad bin Walid Zubaydi, from Luqman bin Amir, from Khalid bin Madan, with his chain, similarly. This is good, for Ismail bin Ayyash is trustworthy in his narrations from the people of his own land, the Syrians, and this is one of them.The hadith has another chain from Abdullah bin Busr himself, in what Ahmad narrated (17686) from Ibrahim bin Ishaq Talaqani, who said, Walid bin Muslim narrated to us, from Yahya bin Hassan, who said, "I heard Abdullah bin Busr say, 'You see this hand of mine, with it I pledged allegiance to the Messenger of Allah ﷺ,' and the Messenger of Allah ﷺ said, 'Do not fast on Saturday, except for what has been made obligatory upon you.'" This is a sahih chain. Yahya bin Hassan is Bakri, Filastini, and he was corroborated.Ibn Hibban narrated it in his Sahih (3615) from Abu Yaala, who said, Hakam bin Musa narrated to us, who said, Mubashshir bin Ismail narrated to us, from Hassan bin Nuh, who said, "I heard Abdullah bin Busr Mazini, a companion of the Messenger of Allah ﷺ, say, 'You see this hand of mine, I pledged allegiance with it to the Messenger of Allah ﷺ,' and I heard him say, 'Do not fast on Saturday except for what has been made obligatory upon you, and if one of you can find nothing but the bark of a tree, let him break his fast with it.'" Its chain is sahih. Hassan bin Nuh is "trustworthy."This chain explicitly states that Abdullah bin Busr heard this hadith directly from the Prophet ﷺ, so perhaps he first heard it from his sister Samma, and then it became possible for him to hear it directly from the Prophet ﷺ, and both are possible. The hadith has other chains, but the ones I have mentioned are the most sahih of them.Once you know this, there is no basis for Nasai's statement that it is an inconsistent hadith, because the principles of hadith science dictate that what is sahih cannot be discredited by what is not sahih.As for the objection based on the hadith of Juwayria, the correct view is that it does not contradict it but rather supports it, because the Prophet ﷺ came to her on a Friday while she was fasting, and he said, "Did you fast yesterday?" She said, "No." He said, "Do you intend to fast tomorrow?" She said, "No." He said, "Then break your fast." This shows the prohibition of fasting Friday alone, and the permissibility of doing so along with the day before it, which is Thursday, or the day after it, which is Saturday. It does not mention the permissibility of fasting Saturday alone, but rather along with the day before it, which is Friday, or the day after it, which is Sunday.Ibn Qayyim discussed at length in Tahzib Sunan the sourcing of this hadith and the explanation of its defects, and the reason for the prohibition of fasting on this day, and he concluded that fasting Saturday alone is disliked, but if one fasts a day before or after it, that is permissible, reconciling between the hadiths. Nawawi said likewise in Sharh Muhazzab.Ibn Mulaqqin said in Badr Munir (5/763), "The truth is that it is a sahih hadith, not abrogated." And success is from Allah.