Sheikh Sayyed Sabiq, states in his well-known book, Fiqh As-Sunnah, in this regard:
1. Acts that nullify ablution
Passing urine, feces or gas
Ablution is nullified by urinating, defecating (Almighty Allah says, “…or one of you comes from relieving himself,” thus proving that such an act obligates a new purification), and releasing gas from the anus. Abu Hurairah reported that the Messenger of Allah (peace and blessings be upon him) said, “Allah does not accept the prayer of a person who has released gas until he makes a new ablution.” A person from Hazhramaut asked Abu Hurairah, “What does ‘releasing gas’ mean?” He answered, “Wind with or without sound.” (Reported by Al-Bukhari and Muslim.) He also quoted the Prophet (peace and blessings be upon him) as saying, “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 Prayer unless he hears its sound or smells its scent.” (Reported by Muslim)
Hearing the escaping gas or smelling it is not a condition for nullifying ablution, but the Prophet (peace and blessings be upon him) stressed that one should be certain of the action.
Sexual discharge from the penis or vagina
As for madhi, which is prostatic fluid in men, and sexual vaginal discharge in women, the Prophet (peace and blessings be upon him) said, “Make ablution” (i.e. one should renew his ablution upon noticing such discharge). Concerning semen or mani, said Ibn `Abbas, “It requires ghusl (bathing) and for madhi and wadi (a thick white secretion discharged (by some people) after urination), wash your sex organs and make wudu’ (ablution).” This was reported by Al-Baihaqi in his Sunan.
Deep sleep that makes a person completely unaware of his surroundings
If a person does not keep his buttocks firmly seated on the floor while sleeping, he must make a new ablution. If one’s buttocks are kept firmly on the floor during one’s sleep, no new ablution is necessary. This is implied by the Hadith of Anas who said, “The Companions of the Prophet (peace and blessings be upon him) were waiting for the late night Prayer until their heads began nodding up and down (from drowsiness and sleep). They would then pray without performing ablution.” (Reported by Ash-Shafii, Muslim, Abu Dawud and At-Tirmidhi). The wording that At-Tirmidhi recorded from the chain of Shubah is, “I have seen the Companions of the Prophet (peace and blessings be upon him) sleeping to the extent that one could hear some of them snoring. But, they would stand for Prayer without a new ablution.” Ibn Al-Mubarak said, “In our opinion, this happened when they were sitting.”
Loss of consciousness
This nullifies the ablution regardless of whether it is due to insanity, fainting, drunkenness, or some medicine. It also does not matter if one is unconscious for a short or long period of time, or if one is sitting, or falls to the ground, and so on. The aspect of unawareness here is greater than that of sleeping. The scholars have unanimous view on this point.
Touching the sexual organ with the bare hand
Busrah bint Safwan narrated that the Prophet (peace and blessings be upon him) said, “Whoever touches his sexual organ cannot pray until he performs ablution.” (Reported by Al-Bukhari) In the narration of Ahmad and An-Nasa’i, Busrah heard the Prophet saying, “Ablution is to be made by a person who touches his sexual organ.” This is general and encompasses touching one’s own sexual organs or touching somebody else’s. Abu Hurairah reported the Prophet (peace and blessings be upon him) as saying, “Whoever touches his sexual organ without any covering (between them) must perform ablution.” (Reported by Ahmad, Ibn Hibban and Al-Hakim, who classified it as sahih (authentic), as did Ibn `Abdul-Barr).
Ibn As-Sakin said, “That Hadith is from the best of what has been related on this topic.” Ash-Shaf’i related: “Any man who touches his penis must perform ablution. Any woman who touches her vagina must perform ablution.” Commenting on its authenticity, Ibn Al-Qayyim quotes Al-Hazimi as saying, “That chain is sahih.”
However, the Hanifi scholars are of the opinion, based on the following Hadith, that touching the sexual organ does not nullify the ablution: “A man asked the Prophet (peace and blessings be upon him) if touching the penis necessitates performing ablution. The Prophet (peace and blessings be upon him) said: “No, it is just a part of you.” (Reported by “the five” (five scholars of Hadith). Ibn Hibban classified it as sahih, and Ibn al-Madini said, “It is better than the Hadith of Busrah.”)
2. Acts that do not nullify ablution
Touching a woman
`A’ishah reported that Allah’s Messenger (peace and blessings be upon him) kissed her while he was fasting and said, “Kissing does not nullify the ablution, nor does it break the fast.” (Reported by Ishaq ibn Rahawaih and Al-Bazzar with a good chain of transmission.)
`A’ishah also said, “One night, I missed Allah’s Messenger in my bed, and so went to look for him. I put my hand on the bottom of his feet while he was praying and saying, ‘O Allah, I seek refuge in Your pleasure from Your anger, in Your forgiveness from Your punishment, in You from You. I cannot praise you as You have praised Yourself.’” (Reported by Muslim and At-Tirmidhi, who classified it as sahih), and she also reported, “The Prophet kissed some of his wives and went to Prayer, without performing ablution.” (Reported by Ahmad and “the four” and its narrators are trustworthy.) She also said, “I would sleep in front of the Prophet (peace and blessings be upon him) with my feet in the direction of the qiblah (to him). When he prostrated, he would touch me, and I would move my feet.” In another narration it says, “When he wanted to prostrate, he would touch my legs.”
Bleeding
This involves bleeding due to a wound, cupping or a nosebleed, and regardless of the quantity of blood.
Al-Hassan says, “The Muslims used to pray even while wounded.” (Reported by Al-Bukhari) He also reported, “Ibn Umar squeezed a pimple until it bled, but he did not renew his ablution. Ibn AbiUqiyy spat blood and continued his Prayer. Umar ibn Al-Khattab prayed while blood was flowing from him.Ibbad ibn Bishr was hit with an arrow while praying, but continued his Prayers.” (Reported by Abu Dawud, Ibn Khuzaimah, and Al-Bukhari.)
Vomit
Regardless of whether the amount of vomit is great or small, there is no authentic Hadith stating that it nullifies ablution.
Having doubts about releasing gas
This is the case where a person is not quite sure whether he is in a state of purity or not. Such a confusing state of mind does not nullify ablution, regardless of whether the person is in Prayer or not, until he is certain that his ablution has been nullified. `Abbad ibn Tameem related that his uncle queried the Prophet (peace and blessings be upon him) about a person who feels something in his abdomen while praying. The Prophet (peace and blessings be upon him) is reported to have said, “He should not leave (the Prayer) until he has heard or smelled it.” (Reported by Muslim, Abu Dawud and At-Tirmidhi). It does not mean that its sound or bad smell nullifies ablution, but that one must be certain of the act that nullifies one’s ablution. Ibn Al-Mubarak said: “If one is uncertain about one’s condition of purity, one does not need to perform a new ablution.” Further, if one is certain that his ablution has been nullified and doubts whether he has purified himself or not, he must perform a new ablution.”