Musnad Abu Ya'la al-Mawsili 140:4

Abu Bakr ibn Abi Shaybah narrated to us, Waki' narrated to us, from al-A'mash, from Yazid al-Raqashi, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Gabriel came to me with a white mirror in which there was a black dot. He said: 'What is this?' He said: 'This is Friday, and in it is an hour.'"