Musnad Ahmad 30:2733

Hajjaj narrated to us, he said: Shareek narrated to us, from Ibrahim ibn Jarir, from Abu Zur'ah, from Abu Hurairah, who said: "When the Prophet, peace and blessings of Allah be upon him, would enter the privy, he would call for water and perform istinja (cleaning himself), then he would wipe his hand on the ground, then he would perform wudu."