Musnad Ahmad 1163:9

Sufyan ibn 'Uyaynah narrated to us, from al-Shaybani, from 'Abdullah ibn Shaddad, from Maymunah: "That the Prophet, peace and blessings of Allah be upon him, prayed while he was wearing a *mirt* (a garment) belonging to one of his wives, and she was wearing some of it." Sufyan said: "I think he said: 'She was menstruating.'"