Musannaf Ibn Abi Shayba 1:1301

Abu Bakr narrated to us, he said: Hushaym narrated to us, he said: Mansur narrated to us, from al-Hasan, he said: Someone who saw the Prophet, may Allah bless him and grant him peace, urinating while sitting narrated to me, and he spread his legs wide until we thought that his hip would become dislocated.