Musnad Ahmad 32:132

Sufyan narrated to us, from al-Zuhri, from Anas, who said: "The last look I took at the Messenger of Allah, may Allah bless him and grant him peace, was on Monday. He pulled back the curtain while the people were behind Abu Bakr, and I looked at his face, and it was as if it were a page of a Mushaf. The people wanted to move, but he signaled to them to remain steady. Then he let the curtain fall, and he passed away at the end of that day, may Allah bless him and grant him peace."