Musannaf Ibn Abi Shayba 37:450

Waki’ narrated to us, from Kahmas ibn al-Hasan, from Ibn Buraydah, from Yahya ibn Ya’mar, from Ibn ‘Umar, from ‘Umar, who said: We were sitting with the Prophet, may Allah bless him and grant him peace, when a man came to him with extremely white clothes and extremely black hair. No trace of travel was seen upon him, and none of us knew him. He drew near to him until he placed his knees against his knees and placed his palms upon his thighs. He said, "O Muhammad, when is the Hour?" He said, "The one questioned about it does not know more than the questioner." He said, "But tell me of its signs: that the slave-girl will give birth to her mistress, and that you will see the barefoot, naked, sheep-herders competing in the construction of buildings."