If a few drops of water fall on a person after completing istinja’, that does not affect tahaarah
Question

                Often times, when I do instinjah, I feel like the water is spilling back on me. What is the ruling on the water that spills on one who uses a modern toilet, when washing oneself. Is the water from the toilet that spills after one releived oneself najis? how to deal with that?.
Praise be to Allah.
      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.