If a person wants to cancel his intention whilst doing wudoo’, praying or fasting

The great scholar al-Sa'di says in al-Fataawa al-Sa'diyyah (p. 228):
Cancelling one's intention with regard to acts of worship is of two
types: one which does not have any effect, which is after the act of
worship has been completed. … The second is cancelling the intention
for the acts of worship when one is still doing it. … This means that
the act of worship is not valid.
Does this mean that if it occurs to me to break an obligatory fast, my
fast becomes invalid? What if that occurs to me but I donot form the
intention to break my fast, is that o.k.?What is the ruling on that?
Similarly in the case of wudoo', if in the middleof it I begin to
doubt whether there is some urine there, for example, then I do not
find any. Sometimes I form the intention to stop doing wudoo', then I
go back and complete it after not finding anything. Should I start
again from scratch because I stopped intending wudoo' in this case?.
Praise be to Allaah.
If a person forms the intention to stop doing anact of worship whilst
he isdoing it, it becomes invalid. No exception is made except in the
case of Hajj and 'umrah, which do not become invalid if the intention
is cancelled or even if a person declares that he is going to stop.
Rather the muhrim remains in ihraam until he has completed the rituals
or he exits ihraam because of being prevented from continuing.
It says in al-Mughni (1/278): If he started his prayer with a sound
intention, then he forms the intention to stop praying and exit from
it, then it becomes invalid. This was the view of al-Shaafa'i. End
quote.
It says in Zaad al-Mustanqi' in the chapter on prayer: If he decides
to end his prayer whilst praying or he becomes hesitant, then his
prayer is invalidated. In the chapter on fasting he said: The one who
forms the intention to break the fast has broken the fast.
But Shaykh Ibn 'Uthaymeen (may Allaah have mercy on him) said in
al-Sharh that hesitatingabout ending the prayer does not make the
prayer invalid. See: al-Sharh al-Mumti' (1/486). An example of
hesitating is if he hears someone knocking at the door, and he wonders
whether he should stop praying or carry on?
Thus it is clear that if a person decides to stop theact of worship,
it becomesinvalid, but if that was simply a thought that crossed his
mind, it does not invalidate the act of worship.
Based on this, if the idea of stopping the fast simply occurs to one,
it does not invalidate the fast until he decides and forms the
intention to break the fast.
Similarly, if whilst doing wudoo' he becomes uncertain as to whether
urine has come out of him, and he pauses and looks, but does not
intendto stop doing wudoo', andhe does not find anything,then his
wudoo' is not invalidated.
Similarly if he intends to stop doing wudoo' his wudoo' is
invalidated, andit is not permissible for him to complete it, rather
he must start all over again.
It says in al-Insaaf (1/151): If he cancels his intention whilst
purifying himself, what he has already done is invalidated, according
to the correct view. This was the view favoured by Ibn 'Aqeel and by
al-Majd in his commentary. And it was said that what has already been
done is not invalidated thereby; this was stated by al-Musannaf in
al-Mughni. End quote.
But one must beware of waswasah, because the shaytaan may come to a
person and make him think that something has come out of him, and the
person may get carried away with that and hardly be able to do any act
of worship without doubting about it, which may cause him hardship and
distress.

And Allaah knows best.