If the blood on it is less than the size of a 'dirham' – which should be considered to be no more than 2.3cm as an obligatory precaution – then it is exempted during the prayer from the rule of najasah, and one may pray with such clothing. However, such clothes are still najis, so if his hand touches the blood on the clothes with transferable dampness, then he must purify the hand before offering the prayer.
