The Messenger of Allah ﷺ said, "Do not fast on Saturday except in what has been made obligatory upon you. If one of you can find nothing but a grape stem or the bark of a tree, let him chew on it."Humayd bin Masadah narrated to us, Sufyan bin Habib narrated to us, from Thawr bin Yazid, from Khalid bin Madan, from Abdullah bin Busr, from his sister, who said, "The Messenger of Allah ﷺ said," and he mentioned something similar.
حدثنا أبو بكر بن أبي شيبة حدثنا عيسى بن يونس عن ثور بن يزيد عن خالد بن معدان
قال رسول الله ﷺ لا تصوموا يوم السبت إلا فيما افترض عليكم فإن لم يجد أحدكم إلا عود عنب أو لحاء شجرة فليمصه حدثنا حميد بن مسعدة حدثنا سفيان بن حبيب عن ثور بن يزيد عن خالد بن معدان عن عبد الله بن بسر عن أخته قالت قال رسول الله ﷺ فذكر نحوه
Its chain is hasan. Abu Dawud narrated it in the chapter on fasting, in the section prohibiting singling out Saturday for fasting (2421), from Humayd bin Masada with this wording. Tirmidhi graded it hasan (744), and Ibn Khuzayma graded it sahih (2164), as did Hakim (1/435), Dhahabi, and Ibn Sakan, and Diya Maqdisi included it in Al-Ahadith Al-Mukhtarah.