3 – Hadith 554 states explicitly that cupping breaks the fast of both the cupper and the person being cupped.

4 – Hadith 555 indicates that cupping is disliked for the fasting person and that, in the earliest period, it invalidated the fast of both parties, after which a concession was granted.

5 – When the early generations describe something as “disliked” (makrūh), they mean “prohibitively disliked,” i.e., forbidden.

6 – The rationale for the one being cupped breaking his fast is that cupping draws blood from the body, causing fatigue and weakness in addition to the weakness already produced by fasting. Out of mercy to His servants, Allah therefore made cupping an act that breaks the fast so that two causes of debility do not coincide for the fasting Muslim.
As for the cupper, the customary method was for him to draw the blood by suction through his cupping instrument; some of that blood could reach his stomach, thereby invalidating his fast.

7 – Practices analogous to cupping in this ruling include venesection and the withdrawal of a large quantity of blood by hospital syringes, because all involve removing blood from the body and so bring about fatigue and weakness that nullify the fast. According to the preponderant view adopted by Shaykh al-Islām, breaking the fast through venesection or significant blood donation follows sound analogy.

8 – A small amount of blood leaving the body—such as that caused by a tooth extraction, drawing a laboratory sample, a minor cut, and the like—does not break the fast.

Disagreement among the scholars
The scholars differed over cupping: does it invalidate the fast or not?
The three Imams maintained that it does not, citing al-Bukhārī’s report from Ibn ʿAbbās: “The Prophet ﷺ underwent cupping while he was fasting and in a state of iḥrām.”
Abū Saʿīd, Ibn Masʿūd, Umm Salamah, and al-Ḥusayn ibn