Musnad Abu Ya'la al-Mawsili 150:286

'Abd al-Rahman ibn Salih narrated to us, Sharik narrated to us, from al-Miqdam ibn Shurayh, from his father, from 'A'ishah, who said: "Whoever tells you that the Prophet, peace and blessings of Allah be upon him, used to urinate while standing, then do not believe him. I saw him urinating while sitting."