Short answer
If the splash came while you were relieving yourself, it is impure: wash it if the total is more than a dirham, and tiny pinpoint splashes are excused. If it came from clean water in the bowl after flushing, it is pure. If you are not sure, treat it as pure. A splash never breaks wudu.
Water mixed with urine or stool is najis; splashes of it on the body or clothes should be washed, and must be washed before prayer if together they cover more than the area of a dirham. Fine spray like the points of needles is excused. The fresh water in a flushed bowl is pure. When in doubt, the thing remains pure.
The four schools agree on this.
Explanation
Splashes in the toilet worry many people. Two principles settle most cases. First, impurity is a fact you know, not something you imagine: the Prophet ﷺ told a person who felt something in his stomach not to leave the prayer unless he heard a sound or noticed a smell. Second, guarding yourself from urine matters, since he warned of punishment in the grave for one who was careless about it.
Cases, for a Hanafi:
- Splash while urinating or passing stool: the water is mixed with impurity. Wash the spot on your skin or clothes. If you pray with it, the prayer is valid only if all such spots together are no larger than a dirham (about the hollow of the palm). See impurity on clothes.
- Fine mist you cannot see: excused, like the points of needles.
- Splash when you flush with nothing in the bowl, or from clean water in the bowl: pure.
- Splash from a bidet, shattaf or jug while washing: the first water that hits the impurity can carry it, so finish your istinja and wash any spots on your legs. Once the area is clean, later splashes are pure.
- A wet toilet seat in a public toilet: assume it is water unless you can see or smell urine.
- Not sure where the splash came from: treat it as pure. Do not inspect.
None of this breaks wudu (ablution). Touching or being splashed with najis only means washing that spot: see touching najis. To cut down splashes, sit or squat, lay a little toilet paper on the water first, and flush with the lid down.
If these thoughts keep coming back, see doubts about najis.
The evidence
Sahih Muslim 362
Sahih... اذا وجد احدكم في بطنه شيئا فاشكل عليه ...
"If one of you feels something in his stomach and is unsure whether anything came out."
The basis of the rule that doubt does not overturn what you are sure of, including purity.
Sahih al-Bukhari 216
Sahih... كان لا يستتر من بوله ...
"He used not to guard himself from his urine."
Shows the importance of protecting body and clothes from urine.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Taharah, chapter on najasat. Splashes of urine like needle points are excused.
- al-Fatawa al-Hindiyya, Kitab al-Taharah, chapter on najasat. Water splashed from the place of istinja and the rule of doubt.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.