What Invalidates Wudu

If any of the following happen, you must perform wudu again before doing salah. It is recommended (but not mandatory) to always be in a state of wudu, so it is not necessary to wait until hearing the adhan (call to Salah) before performing wudu.

1. Relieving oneself from the call of nature

O you who believe! When you intend to offer As-Salât (the prayer), wash your faces and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to ankles. If you are in a state of Janâba (i.e. had a sexual discharge), purify yourself (bathe your whole body). But if you are ill or on a journey or any of you comes from answering the call of nature, or you have been in contact with women (i.e. sexual intercourse) and you find no water, then perform Tayammum with clean earth and rub therewith your faces and hands. Allâh does not want to place you in difficulty, but He wants to purify you, and to complete His Favour on you that you may be thankful. [The Noble Qur'an 5:6]

2. Passing gas, i.e. breaking wind

• Abu Hurairah reported that the Messenger of Allah said, "Allah does not accept the prayer of a person who has released gas until he makes a new ablution." A person from Hadhramaut asked Abu Hurairah, "What does releasing gas mean?" He answered, "Wind with or without sound." [Related by al-Bukhari and Muslim.]
• Abu Hurairah also narrated that the Prophet said, "If one of you finds a disturbance in his abdomen and is not certain if he has released any gas or not, he should not leave the mosque unless he hears its sound or smells its scent." [Related by Muslim.]
• 'Abbad ibn Tameem related that his uncle queried the Prophet about a person who feels something in his abdomen while praying. Said the Prophet , "He should not leave (the prayer) until he hears it or smells it." [Related by Muslim, Abu Dawud and at-Tirmidhi.]

3. Sexual Relations

One must perform ghusl after sexual relations, i.e. when the male private part enters the female private part.

4. Touching one's private parts with no barrier (i.e. cloth) between the hand and organ

Busrah bint Safwan narrated that the Prophet said, "Whoever touches his sexual organ cannot pray until he performs ablution." [This hadith is related by "the five." At-Tirmidhi classified it as sahih and al-Bukhari called it the most authentic report on that topic. Malik, ash-Shaifi, Ahmad and others also narrated it.]

In the event that you forget whether you have done wudu or not, err on the side of not re-performing wudu.... i.e. do not perform wudu again unless you are sure that you broke/invalidated it.

Make wudu again if you experience anger.

Hadith - Dawud, Narrated Atiyyah as-Sa'di

AbuWa'il al-Qass said: We entered upon Urwah ibn Muhammad ibn as-Sa'di. A man spoke to him and made him angry. So he stood and performed ablution; he then returned and performed ablution, and said: My father told me on the authority of my grandfather Atiyyah who reported the Apostle of Allah (peace be upon him) as saying: Anger comes from the devil, the devil was created of fire, and fire is extinguished only with water; so when one of you becomes angry, he should perform ablution.