Musnad Ahmad 32:1005

Rawh ibn ‘Ubadah narrated to us, Hajjaj ibn Hassan narrated to us, he said: We were with Anas ibn Malik, and he called for a vessel in which there were three iron rivets and an iron ring. He brought out from a black sheath [something] that was less than a quarter and more than half of a quarter. Anas ibn Malik ordered that water be put into it for us. It was brought to us, so we drank from it and poured it over our heads and our faces, and we invoked blessings upon the Prophet, may Allah bless him and grant him peace.