Reference: al-Muntaqaa min Fataawa Shaykh Saalih al-Fowzaan – Volume 5, Page 8, Fatwa No.3
Question: We have at work inside the toilets what they call a urinal, [whereby] some of the brothers who wear trousers come and urinate whilst standing. So how can they guarantee that none of the urine will fall on their trousers? One day I advised an individual [against urinating whilst standing], and he said: The Messenger (sal-Allaahu ‘alayhi wa sallam) has not prohibited us from [doing] that. I request [from you] advice and guidance?
Response: It is permissible for someone to urinate whilst standing so long as he safeguards his body and clothes from the sprinkles of the urine, since the Prophet (sal-Allaahu ‘alayhi wa sallam) urinated whilst standing on some occasions [refer to Saheeh al-Bukhaaree ref: 1/26 the hadeeth of Hudhayfah (radhi-yAllaahu ‘anhu)]. This is particularly the case if he needs to do so because of the tightness of his clothes or a pain in his body, otherwise it is disliked if done without any [genuine] need.