بسم الله الرحمن الرحیم
It is makrūh (disliked) to urinate in a standing position without an excuse as it is against the Sunnah method (al-Baḥr al-Rāʾiq 1: 256; Marāqī al-Falāḥ, p. 27; Radd al-Muḥtār, 1: 344).
The mother of the believers, ʿĀʾishah (may Allah be pleased with her) is reported to have said: “Whosoever narrates to you that the Prophet ﷺ would urinate whilst standing, then do not believe him. He would not urinate except in a sitting position” (Sunan al-Tirmidhī, 12; Sunan al-Nasāʾī, 29).
Once, the Prophet ﷺ saw ʿUmar (may Allah be pleased with him) urinating standing up so he said, “O ʿUmar, do not urinate standing up”. ʿUmar says that after this he never urinated whilst standing (Sunan al-Tirmidhī, 12; Muṣannaf ʿAbd al-Razzāq, 15924; Sunan Ibn Mājah, 308).
However, if one has a valid reason, it is permissible to urinate whilst standing, as the Prophet ﷺ is recorded to have done so on at least one occasion (Ṣaḥīḥ al-Bukhārī, 224; Ṣaḥīḥ Muslim, 273).
Allah knows best
17 Rabīʿ al-Awwal 1427 / 15 April 2006