Silsilah as-Sahihah 888 — Ruling on fasting 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)."