Musannaf Ibn Abi Shayba 1:1611

Abu Bakr narrated to us, he said: Hafs ibn Ghiyath narrated to us, from Yahya ibn Sa'id, from Muhammad ibn Yahya ibn Habban, from his paternal uncle Wasi' ibn Habban, from Ibn Umar, he said: I saw the Prophet, may Allah bless him and grant him peace, sitting to relieve himself, facing towards the Qibla.