Reference: I’laam al-Mu’aasireen bi-Fataawa Ibn ‘Uthaymeen – Page 41
Question: Is a menstruating woman required to change her clothing after she has [completed her menses and] purified herself, keeping in mind that no blood or impurity has fallen on her clothing?
Response: She is not required to do so. This is because the menses do not make the body impure, rather the blood of the menses makes impure whatever it touches; Because of this, the Prophet (sal-Allaahu ‘alaye wa sallam) ordered the women whose clothing had any [menses] blood on them to wash them and [then] pray in them. [Transmitted by al-Bukhaaree – 227, and Muslim – 110].