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 اللهِ ﷺ قَالَ لَا تَصُومُوا يَوْمَ السَّبْتِ إِلَّا فِيمَا افْتُرِضَ عَلَيْكُمْ فَإِنْ لَمْ يَجِدْ أَحَدُكُمْ إِلَّا عُودَ عِنَبٍ أَوْ لِحَاءِ شَجَرَةٍ فَلْيَمْضُغْهَا  

[Machine] She entered upon the messenger of Allah while he was having his meal, and that was on the day of Saturday. He said, "Come and eat." She replied, "I am fasting." He asked her, "Did you fast yesterday?" She replied, "No." He said, "Then eat, for fasting on the day of Saturday is neither obligatory upon you nor recommended."  

أَنَّهَا دَخَلَتْ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ يَتَغَدَّى وَذَلِكَ يَوْمَ السَّبْتِ فَقَالَ تَعَالَيْ فَكُلِي فَقَالَتْ إِنِّي صَائِمَةٌ فَقَالَ لَهَا صُمْتِ أَمْسِ؟ فَقَالَتْ لَا قَالَ فَكُلِي فَإِنَّ صِيَامَ يَوْمِ السَّبْتِ لَا لَكِ وَلَا عَلَيْكِ  

[Machine] From the Prophet ﷺ: "None of you should fast on Saturday unless it is an obligatory fast. But if one cannot find anything to break his fast except the bark of a tree, then he should break his fast with it."  

عَنِ النَّبِيِّ ﷺ قَالَ لَا يَصُومَنَّ أَحَدُكُمْ يَوْمَ السَّبْتِ إِلَّا فِي فَرِيضَةٍ وَإِنْ لَمْ يَجِدْ إِلَّا لِحَاءِ شَجَرَةٍ فَلْيُفْطِرْ عَلَيْهِ