Doubting the nullification of Tahaarah does not nullify ablution
21-1-2014 | IslamWeb
Question:
I often have doubts, whether before or during the prayer, that I have emitted pre-ejaculatory fluid; what should I do?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
If a person is in the state of Tahaarah and then has doubt about that, the basic rule is that he remains in the same state of Tahaarah, and does not move from it to the state of minor impurity without certainty (of that), since the basic rule requires that the current state should remain the same (so long as there is no certainty of it being changed), and in no way could certainty be removed by doubt. A similar complaint was raised to the Messenger of Allaah, sallallaahu ‘alayhi wa sallam, that one imagined to have passed wind in the prayer, thereupon the Prophet, sallallaahu ‘alayhi wa sallam, said: "He should not turn away unless he hears a sound or smells something." [Al-Bukhari and Muslim]
An-Nawawi 
said:
That is, to know, for certain, that one of both has taken place; and it is not a requisite, according to the consensus of scholars, to hear a sound and smell something at the same time. This Hadeeth is one of the principles of Islam, and one of the great juristic rules, i.e. things are judged to remain in their original state until otherwise is certain, and the occurring doubt does not harm. One of those is the case of the chapter in connection with which the Hadeeth was brought, i.e. whoever is certain of being in the state of Tahaarah, and then has doubt about his moving to the state of minor impurity, should then be judged to remain in the same state of Tahaarah; and it does not matter whether this doubt takes place during or outside the prayer. That is the opinion adopted by our school of Fiqh, and the opinion of the majority of the late and early scholars.
Based on that, do not take your doubt about emitting pre-ejaculatory fluid into account. But once you are certain that it has come out, it becomes incumbent upon you to wash the affected part of your body and garment, and ablution is due on you, and the prayer is invalid if it comes out during it.
Allaah Knows best.