Is increasing the ablution – such as washing a limb four times – reprehensible, or does it completely invalidate the ablution, based on the Hadith: "Whoever innovates something in this matter of ours that is not from it, it is rejected"?

The Sunnah in ablution is to wash each limb three times (tathleeth). Anything beyond that is considered an transgression and an injustice, and scholars agree on its reprehensibility. This excess does not invalidate the ablution, because it is valid with three washes, and the additional wash is rejected. As for someone who intentionally adds a fourth wash, they have committed a reprehensible act. But if someone adds it out of forgetfulness, doubt, or ignorance, there is no blame on them. The person afflicted with obsessive doubts (al-waswas) is often excused, but they must resist these doubts, for the one afflicted with waswas is doing wrong and transgressing by exceeding what is prescribed.