If a few drops of water fall on a person after completing istinja’, that does not affect tahaarah
With regard to all religious matters, the Muslim should not pay attention to whatever may happen to him of whispers from the Shaytaan. He has to purify himself as he has been commanded and pray as he has been commanded, and he should not pay any attention to any whispers that the Shaytaan may present to him, aiming to trick him and make him dislike worship and have doubts about it.
These drops that fall from behind after cleaning oneself with water (istinja’) are just the leftovers of the istinja’ water that drip from the body. So you should not pay any attention to that. Go ahead and pray, for the certainty of having purified oneself cannot be cancelled out by mere doubt. This in fact is a kind of waswaas (whisper from the Shaytaan) that must be warded off and given no attention. Al-Bukhaari (2056) and Muslim (361) narrated that ‘Abd-Allah ibn Zayd (may Allah be pleased with him) said: A complaint was made to the Prophet (blessings and peace of Allah be upon him) about when a man thinks that something has happened whilst he is praying. He said: “Do not stop until you hear a sound or notice a smell.”
Al-Nawawi (may Allah have mercy on him) said:
This hadeeth represents one of the basic principles of Islam and one of the most important foundations of fiqh, which is that things are judged to remain as they originally were until there is certainty to the contrary, and fleeting doubts do not affect them. End quote.
Sharh al-Nawawi ‘ala Muslim, 4/49
Shaykh al-Islam (may Allah have mercy on him) was asked: What if a person does wudoo’ and starts to pray, then feels a drip during his prayer: is his prayer invalidated or not?
He replied: Merely feeling something does not invalidate wudoo’ and it is not permissible for him to stop an obligatory prayer because of a mere doubt. But if he becomes certain that urine has escaped to the exterior of the private part, then his wudoo’ is invalidated and he has to clean himself with water (istinja’), unless he suffers from urinary incontinence, in which case that does not invalidate his prayer if he has done what he is enjoined to do.
End quote. Majmoo‘ al-Fataawa, 21/219-220
Shaykh Ibn Baaz (may Allah have mercy on him) said:
The believer should not pay any attention to this whisper, because that will prompt the Shaytaan to make him doubt more, and the Shaytaan is keen to spoil the deeds of the sons of Adam, such as prayer and other things. So it is essential to beware of his plots and whispers, and put one’s trust in Allah and assume that whatever may happen to him of whispers are from the Shaytaan, so that he will not pay attention to it. But if something comes out of him for certain, and there is no doubt, he should clean himself again and do wudoo’. But if there is still some doubt, even if it is a little, then he should not pay any attention to that, so he should continue to believe that his wudoo’ is valid and continue to fight the Shaytaan. End quote.
Majmoo‘ Fataawa Ibn Baaz, 10.123
And Allah knows best.