All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Ablution is invalidated by touching some special parts of the body of the human being, and these parts are specified and mentioned in the books of the scholars; here are the details:
1- Ablution is invalidated for a man if he touches his penis, according to the view of the majority of the scholars. The Fiqh Encyclopedia reads: “The Maaliki, Shaafi'i, and Hanbali Schools are of the view that if a man touches his penis with his hand, then this invalidates ablution, as the Prophet said: “Whoever touches his private part, let him perform ablution.” [End of quote]
2- If a man or a woman touches his/her anus: this matter is a matter of difference of opinion among the scholars. The view which we consider to be the preponderant one is that it invalidates ablution. Ibn Qudaamah said: “With regard to touching the anus, there are also two narrations about this from Imam Ahmad: one of which is that it does not invalidate ablution. This is also the view held by Maalik...The second narration: it invalidates ablution: this was reported by Abu Daawood. This is also the view of ‘Attaa’, Az-Zuhri, and Ash-Shaafi'i: for the general saying of the Prophet “Whoever touches his private part, let him perform ablution.” The anus is one of the two private parts; it is like the penis.” [End of quote]
3- A woman touching her vagina; this is the view of the Maaliki, Shaafi'i and Hanbali Schools.
4- The touching that invalidates ablution is that which is done directly (with the skin) without any barrier, and that it must be with the hand. Ibn Qudaamah said: “His ablution is invalidated only if he touches his penis without an barrier.” [End of quote]
5- Ablution is not invalidated if one touches other than what we have mentioned above. Ibn Qudaamah said: “Ablution is not invalidated if one touches any part of the body other than the two private parts.”
Allah knows best.