Musnad Ahmad 27:170

Yahya narrated to us, from Ubaydullah, Muhammad ibn Yahya narrated to us, from his paternal uncle, from Ibn Umar, who said: I climbed onto the roof of Hafsa’s house one day, and I saw the Messenger of Allah (peace and blessings of Allah be upon him) attending to his need, with his back toward the house and his face toward Ash-Sham.