Musnad Ahmad 534:19

'Ali ibn 'Ayyash narrated to us, he said: Hassan ibn Nuh al-Homsi narrated to us, he said: I saw 'Abdullah ibn Busr saying: "Do you see this palm of mine? I bear witness that I placed it upon the palm of Muhammad, may Allah bless him and grant him peace." And he forbade fasting on Saturday, except for an obligatory fast, and he said: "If one of you finds nothing but the bark of a tree, let him break his fast with it."