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According to the Hanbali school, letting blood out by means of cupping and the like, invalidates the fast. The evidence is in the following hadith:
The Prophet (pbuh) said: “The cupper and the one to whom cupping is done both break their fast.” [ Abu Dawood]
However, according to the majority of scholars, cupping does not invalidate the fast because it was reported that the Prophet (pbuh) did it while he was fasting so they combined both incidents by saying it is only disliked during fasting.