Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 601
Ibn 'Umar said: 'Umar asked the verdict of the Shari'ah from the Apostle (may peace be upon him) thus: Is it permissible for any one of us to sleep in a state of impurity? He (the Holy Prophet said: Yes, he must perform ablution and then sleep and take a bath when he desires.