Musnad Ahmad 1192:3

Al-Hakam ibn Nafi’ narrated to us, he said: Isma’il ibn ‘Ayyash narrated to us, from Muhammad ibn al-Walid al-Zubaydi, from Luqman ibn ‘Amir, from Khalid ibn Ma’dan, from ‘Abdullah ibn Busr, from his sister al-Samma’, from the Prophet, peace and blessings of Allah be upon him, who said: "None of you should fast on Saturday except for what has been made obligatory upon you. And if one of you finds nothing but the bark of a tree, let him break his fast with it."