Musannaf Ibn Abi Shayba 1:1315

Yahya ibn Sa'id narrated to us, from Muhammad ibn Abi Yahya, from 'Umar ibn 'Abd al-Rahman, who said: I saw Sa'id ibn al-Musayyib urinating while standing. So I said, "O Abu Muhammad, you urinate while standing? Do you not fear that it will splash upon you?" He said to me, "Do you not urinate while standing?" I said, "No." He said, "That is worse for you."