Musnad Ahmad 1153:1027

Waki’ narrated to us, from Sufyan, from al-Miqdam, from his father, from ‘Aishah, who said: Whoever tells you that the Messenger of Allah (peace and blessings of Allah be upon him) urinated while standing, do not believe him. The Messenger of Allah (peace and blessings of Allah be upon him) did not urinate while standing since the Quran was revealed to him.