Musnad Ahmad 1153:1578

Waki’ narrated to us, from Sufyan, from al-Miqdam ibn Shurayh ibn Hani’, from his father: ‘A’ishah 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 Qur’an was revealed to him."