Musnad Abu Ya'la al-Mawsili 124:3

Abu Kurayb Muhammad ibn al-Ala al-Hamdani narrated to us, Umar ibn Ubayd al-Tanafisi narrated to us, from Umar ibn al-Muthanna, from Ata al-Khurasani, from Anas ibn Malik, who said: I was with the Messenger of Allah, may Allah bless him and grant him peace, on a journey. He lagged behind for his need, then he came and said, "Is there any water?" I brought him a small water skin (idawah) of water. He performed ablution with it, then he wiped over his leather socks (khuffayn).