Musnad Ahmad 1192:0

Narrated to us by Abd al-Rahman, from Sufyan, from Abu Ishaq, from al-Aswad ibn Yazid, from al-Summa’ bint Busr, that the Messenger of Allah (peace be upon him) said: "Do not fast on Saturday except in what has been made obligatory upon you. And if one of you finds nothing but a grape skin or a piece of a tree branch, let him chew it."