Musnad Ahmad 1192:1

Abu Asim narrated to us, he said: Thawr narrated to us, from Khalid ibn Ma'dan, from Abdullah ibn Busr, from his sister, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Do not fast on Saturday except in what has been made obligatory upon you. If one of you finds nothing but a grape stem or the bark of a tree, let him chew it."