Musnad Ahmad 30:1679

Khalaf narrated to us, he said: Khalid narrated to us, from Suhayl ibn Abi Salih, from his father, from Abu Hurayrah, who said: "The Messenger of Allah, peace and blessings of Allah be upon him, commanded us to cover the vessel of ablution, to tie the mouth of the waterskin, and to turn the container upside down."