The Prophet ﷺ said: Do not fast on Saturday except what has been made obligatory on you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.  

أَنَّ النَّبِيَّ ﷺ قَالَ لاَ تَصُومُوا يَوْمَ السَّبْتِ إِلاَّ فِيمَا افْتُرِضَ عَلَيْكُمْ وَإِنْ لَمْ يَجِدْ أَحَدُكُمْ إِلاَّ لِحَاءَ عِنَبَةٍ أَوْ عُودَ شَجَرَةٍ فَلْيَمْضُغْهُ