Musnad Abu Ya'la al-Mawsili 219:19

Yahya ibn Ayyub narrated to us, Ismail ibn Ayyash narrated to us, he said: Ata al-Khurasani informed me, from Muawiyah ibn Abi Sufyan, who said: I entered upon Umm Habibah, the wife of the Prophet, may Allah bless him and grant him peace, and I saw the Prophet, may Allah bless him and grant him peace, standing, praying in a single garment, having crossed its two ends over one another. I said, "O Umm Habibah, does the Prophet, may Allah bless him and grant him peace, pray in a single garment?" She said, "Yes, and it is the garment in which that which was—meaning sexual intercourse—had taken place."