It is narrated from Hazrat Aisha that sometimes the Prophet (peace be upon him) would be in a state of impurity by choice during the night. Hazrat Bilal would come to inform about the Fajr prayer, so the Prophet (peace be upon him) would get up and perform ghusl, and I would see water dripping from his skin and hair, and I would hear his recitation, and on that day the Prophet (peace be upon him) would be fasting.