Ubaid A’raj says: My grandmother narrated to me that she went to the Messenger of Allah (peace and blessings be upon him) while he was having lunch; it was a Saturday. The Prophet (peace and blessings be upon him) said:“Come, eat food.”She said: I am fasting. The Prophet (peace and blessings be upon him) said:“Did you fast yesterday?”She said: No. The Prophet (peace and blessings be upon him) said:“Then eat food, for you will get neither reward nor punishment for fasting only on Saturday.”

It is narrated from Sayyiduna Abu Umamah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "Do not fast on Saturday, except for obligatory fasts. If you find nothing but the bark of a tree, then eat that (to show that you are not fasting)."