عَنْ عَبْدِ اللَّهِ بْنِ بُسْرٍ رَضِيَ اللَّهُ عَنْهُ عَنْ أُخْتِهِ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَآلِهِ وَسَلَّمَ قَالَ: ((لَا تَصُومُوا يَوْمَ السَّبْتِ إِلَّا فِيمَا افْتُرِضَ عَلَيْكُمْ، فَإِنْ لَمْ يَجِدْ أَحَدُكُمْ إِلَّا عُودَ عِنَبٍ أَوْ لِحَاءَ شَجَرَةٍ فَلْيَمْضَغْهُ))
It is narrated from the sister of Sayyiduna Abdullah bin Busr (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: Do not fast on Saturday, except for those days on which fasting has been made obligatory upon you. If someone has nothing to eat on that day except a stick of grapevine or the bark of a tree, then he should chew that.