Hadith #2421 (Sunan Abi Dawud)
Book: Fasting (Kitab Al-Siyam)
Narrator(s): Abu Ayyub al-Ansari, Hafs bin 'Umar bin al-Harith حفص بن عمر بن الحارث, Hmam bin Yahya bin Dinar همام بن يحيى بن دينار, Juwayriya bint al-Harith, Muhammad bin Kathir bin Abi 'Ata' محمد بن كثير بن أبي عطاء الثقفي, Qatada قتادة
Grade: Sahih

The Prophet (ﷺ) said: Do not fast on Saturday except what has been made obligatory on you; and if one of you can get nothing but a grape skin or a piece of wood from a tree, he should chew it.

Source: https://parallelquran.com/hadith/sunan-abi-dawud-2421/