The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, commented: "If blood touches his clothing and its amount is the size of a dirham wafiy (equivalent to a dirham and a third), he must wash it, and it is not permissible for him to pray in it. If the amount is less than that, such as the size of a lentil or a fingernail, it is permissible for him to pray in it before washing it, although washing it for prayer is preferable. However, if the blood is menstrual blood, then it is not permissible to pray in the garment, whether the amount is small or large. Washing the garment is obligatory, even if the amount is as small as the tip of a needle."
Commentary #4