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.
Hadith Referenceالفتح الربانی / كتاب الصيام / 3878