Jami` at-Tirmidhi 744
جامع الترمذي#744SahihAbdullah bin Busr narrated from his sister that :the Messenger of Allah said: "Do not fast on Saturday except for what has been made obligatory upon you (by Allah). If one of you does not find but a grape peal or a tree's twig, then let him chew it
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Bashar Awad MaaroufHasan
Zubair Ali ZaiIsnaad Hasan
In-book reference: 8:63