Musnad Ahmad 1192:2

Yahya ibn Ishaq narrated to us, he said: Ibn Lahi'ah informed us, he said: Musa ibn Wardan informed us, from Ubayd al-A'raj, who said: My grandmother narrated to me that she entered upon the Messenger of Allah (peace and blessings of Allah be upon him) while he was eating lunch, and that was on a Saturday. He said: "Come and eat." She said: "I am fasting." He said to her: "Did you fast yesterday?" She said: "No." He said: "Then eat, for fasting on Saturday is neither for you nor against you."