Fasting (Kitab Al-Siyam)
Dawud :: Book 13 : Hadith 2415
Narrated As-Samma' sister of Abdullah ibn Busr:
The Prophet (peace_be_upon_him) said: Do not fast on Saturday except what has been made obligatory on 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.