May a person who urinated perform ablution before waiting for a certain period?
After urinating, more or less leakage is possible depending on the person, conditions, and even the age. It would be more appropriate to wait for a while so that this leakage ends fully. This waiting period of time is called istibra (Ibn Abidin, Radd al-Muhtar, I, 558).
Istibra may be performed by walking, coughing, moving, etc.
If a person performs ablution after urinating without performing istibra and discharge comes after ablution, his ablution is nullified and he will have to perform ablution again (Marghinani, al-Hidayah, I, 106-107). Moreover, if cleaning is not done well the leakage of urine will contaminates the clothes and if it reaches a certain extent (covers a part as big as palm of a hand) it prevent establishing a valid prayer (Mawsili, al-Ikhtiyar, I, 110-111).
It is permissible for those, who do not have such leakage problem after urinating, to perform ablution right away without waiting. People who show necessary sensitivity on the issues of istibra and cleaning should not exaggerate it and pay attention to unnecessary anxiety or misgivings.
Source: Presidency Of Religious Affairs The Turkey, High Board of Religious Affairs FATWAS