Khalid bin Ma'dan (may Allah have mercy on him) relates that Sayyiduna Abdullah bin Busr (may Allah be pleased with him) told him that he heard his father, Sayyiduna Busr (may Allah be pleased with him), say: The Messenger of Allah (peace and blessings be upon him) forbade fasting on Saturday, and he said: "If one of you finds nothing (to eat) except chewing the bark of a tree, still let him not fast on that day." And Sayyiduna Abdullah bin Busr (may Allah be pleased with him) said: If you have any doubt, then ask my sister. So Khalid bin Ma'dan went to her and asked her about what Abdullah had related, and she related the same to him.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1180