Musannaf Ibn Abi Shayba 1:1323

Abu Bakr narrated to us, he said: Sharik narrated to us, from al-Miqdam ibn Shurayh, from his father, from Aisha, she said: "Whoever tells you that the Messenger of Allah (peace and blessings of Allah be upon him) urinated while standing, do not believe him. I saw him urinating while sitting."