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.