Praise be to Allaah.
There is nothing wrong with that. The imaams of the salaf and the scholars allowed that. Ibn al-Mundhir narrated in al-Awsat that there was consensus on this matter. He said, “There is no apparent sense in not allowing that, because the water is pure and the body of the man is pure, so the water coming out of his body is also pure, therefore it does not contaminate the place. But I regard it as makrooh to do wudoo’ in the place where people pray lest the splashing of the water disturb the people.
It was narrated that Ibn ‘Abbaas said that there is nothing wrong with that. This was narrated by Ibn al-Mundhir in al-Awsat.
Ibn Abi Shaybah and Ibn al-Mundhir narrated that Ibn ‘Umar (may Allaah be pleased with them both) used to do wudoo’ in the mosque.
The issue of wudoo’ does not include the issue of relieving oneself. Relieving oneself in the place where people pray, whether by urinating or defecating, is haraam by scholarly consensus.