The son of Abdullah bin Busr (may Allah be pleased with him) narrates from his father, and he from his paternal aunt al-Samma' (may Allah be pleased with her), that the Messenger of Allah (peace and blessings be upon him) forbade fasting on Saturday and said: "If any of you finds nothing but a green stick, let him break his fast with it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8494