Mishkat al-Masabih 7 Fasting كتاب الصوم 106

وَعَنْ عَبْدِ اللَّهِ بْنِ بُسْرٍ عَنْ أُخْتِهِ الصَّمَّاءِ أَنَّ رَسُولَ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ قَالَ: «لَا تَصُومُوا يَوْمَ السَّبْتِ إِلَّا فِيمَا افْتُرِضَ عَلَيْكُمْ فَإِنْ لَمْ يَجِدْ أَحَدُكُمْ إِلَّا لِحَاءَ عِنَبَةٍ أَوْ عُودَ شَجَرَةٍ فَلْيَمْضُغْهُ» . رَوَاهُ أَحْمَدُ وَأَبُو دَاوُدَ وَالتِّرْمِذِيُّ وَابْنُ مَاجَهْ والدارمي

‘Abdallah b. Busr, on the authority of his sister as-Samma’, reported God’s messenger as saying, “Do not fast on Saturday except during what has been made obligatory for 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.” Ahmad, Abu Dawud, Tirmidhi, Ibn Majah and Darimi transmitted it.