The things that nullifies ablution.

 The things that nullifies ablution. 

Alhamdulillah, Ablution is a minor purity and as reported in hadith, 

When one makes intention to make ablution and as he wash his hands, the sins from his hands are removed and when he washes his mouth, the sins he commits with his mouth is washed away

As reported in hadith, 

Abu Ayyub, Jabir and Anas said that when this verse came down, “In it are men who love to be purified, and God loves those who purify themselves,”* God’s messenger said, “God has praised you Ansar for purification. What does your purification consist of?” They replied, “We perform ablution for prayer, wash after seminal emission, and cleanse ourselveswith water.” He said, “That is it, so keep on doing it.” *1. Qur'an, ix, 108. Ibn Majah transmitted it.

Mishkat al-Masabih 369.

Humran, the freed slave of 'Uthman reported:

I brought for Uthman b. 'Affan the ablution water. He performed ablution and then said: Verily the people narrate from the Messenger of Allah (ﷺ) a hadith. I do not know what these are. but (I know this fact) that I saw the Messenger of Allah (ﷺ) perform ablution like this ablution of mine and then he said: He who performed ablution like this, all his previous sins would be expiated and his prayer and going towards themosque would have an extra reward. In the tradition narrated by Ibn 'Abda (the words are):" I came to Uthman and he performed ablution."

Sahih Muslim 229.

The things that nullifies the ablution,

Narrated ‘Aisha (rad)::

Allah’s Messenger (ﷺ) said: “Whoever vomits, bleeds through the nose, or released Madhi (urethral discharge) should go, perform ablutionand then complete his Salat (prayer) (by continuing from where he had stopped at) on condition that he does not speak in the process”. Reported by Ibn Majah

Bulugh al-Maram 74.

From this hadith, the last word should continue his prayer after the ablution and not starting it afresh, scholars say it was not word of Ayesha R.A

And when one touches his private part out of desire should make up his ablution again. 

As from the hadith, 

Narrated Busra bint Safwan (rad)::

Allah’s Messenger (ﷺ) said: “He who touches his penis should perform ablution”. [Reported by Al-Khamsa, and At-Tirmidhiand Ibn Hibban graded it Sahih (sound).

Bulugh al-Maram 73.

Also deep sleeping nullifies the ablution where one lies and does not know what is happening. 

As reported in the hadith, 

And Ibn ‘Abbas (rad) narrated in a Marfu’ (traceable) Hadith that:

Ablution is necessary for one who sleeps while he is lying flat. 

Bulugh al-Maram 82.