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." (Using translation from Tirmidhī 744)   

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «لَا تَصُومُوا يَوْمَ السَّبْتِ إِلَّا فِيمَا افْتُرِضَ عَلَيْكُمْ وَإِنْ لَمْ يَجِدْ أَحَدُكُمْ إِلَّا كَذَا أَوْ لِحَاءَ شَجَرَةٍ فَلْيَمْضَغْهُ»