Does touching the anus during ablution (istinja') nullify wudu', or is touching it for the purpose of cleaning not considered a nullifier?

If stool exits, ablution is invalidated. It is not necessary to ensure water reaches the anus, but rather a strong presumption that the impurity has been removed is sufficient. Touching the anus after purification (istinja') is a matter of scholarly disagreement. Some scholars hold that it does not invalidate ablution, while others believe it does, based on the generality of the hadith: "Whoever touches his private part should perform ablution."