Al-Nawawi said: The ummah is unanimously agreed that it is haraam for a menstruating woman to pray, whether that is obligatory (fard) prayers or supererogatory (naafil). Al-Majmoo', 2/351.
Shaykh Ibn Uthaymeen was asked about that and replied:
It is not permissible for a woman who is menstruating or bleeding following childbirth (nifaas) to pray, because the Prophet (peace and blessings of Allaah be upon him) said concerning women: Is it not the case that if a woman menstruates she does not pray or fast? The Muslims are unanimously agreed that it is not permissible for a menstruating women to fast, or for her to pray. Based on this, if a woman does that she has to repent to Allaah and to seek His forgiveness for what she did.
Fataawa al-Mar'ah al-Muslimah, 1/285.
Reproduced from Islam QA