Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.019A.

Section : One may not urinate while standing.

If a person can guarantee that no impurities will touch his clothes, it is permissible to urinate while standing. Said ‘Aishah, “If someone relates to you that the Messenger of Allah urinated while standing, do not believe him. He only urinated while sitting.” This hadith is related by “the five,” except for Abu Dawud. At-Tirmizhi’s comment is, “It is the best thing related on this point, and it is the most authentic.”

One should not forget that what ‘Aishah said is based on the knowledge that she had. Huzhaifah relates that the Messenger of Allah, upon whom be peace, went to a public garbage dump and urinated while standing. Huzhaifah went away, and the Prophet then called him over. The Prophet made ablution and wiped over his shoes. This is related by “the group.”

Commenting upon the issue, an-Nawawi says, “To urinate while sitting is most desirable in my opinion, but to do so standing is permissible. Both acts are confirmed by the Messenger of Allah, upon whom be peace.