What is the authenticity of the Hadith: "Do not fast on Saturday, even if it is the Day of 'Arafah"?

There is no hadith with this exact wording. However, there is a prohibition against singling out Saturday for fasting, unless it is an obligatory fast. This prohibition is one of dislike (karaha), except if it coincides with the Day of Arafah, in which case it is not disliked. The hadith is: "Do not fast on Saturday unless it is that which Allah has made obligatory upon you. If one of you finds nothing but the peel of a grape or a stick from a tree, let him chew it." Narrated by Al-Tirmidhi.