Sahih Muslim
The Book of Menstruation (Kitab Al-Haid)
Book Number: 003 / Hadith Number: 732
Anas b. Malik reported: (The people) stood up for prayer and the Apostle of Allah (may peace be upon him) was talking in whispers with a man, and he did not discontinue the conversation till his Companions dozed off; he then came and led the prayer.