What is the meaning of the hadeeth “Leave that which makes you doubt for that which does not make you doubt”?

This hadeeth was narrated by at-Tirmidhi
(2442), Ahmad (1630) and Ibn Hibbaan (722) from al-Hasan ibn ‘Ali (may Allah
be pleased with him) who said: I memorized from the Messenger of Allah
(blessings and peace of Allah be upon him): “Leave that which makes you
doubt for that which does not make you doubt, for truth leads to reassurance
and lies lead to uncertainty.” Classed as saheeh by Shaykh al-Albaani (may
Allah have mercy on him). 

It was also narrated by an-Nasaa’i
(5614) without the additional words “for truth leads to reassurance…” 

Al-Mannaawi (may Allah have
mercy on him) said: “Leave that which makes you doubt” means: refrain from
that which you are uncertain as to whether it is good or bad, halaal or
haraam

“for that which does not make
you doubt” that is: turn instead to that about which you are not uncertain,
that which you are sure is good and halaal.

“for truth leads to reassurance”
means: the heart is at ease with it

“and lies lead to uncertainty”
means: they make the heart anxious and uneasy. At-Teebi said: This phrase
explains the preceding words. What it means is: if you find yourself
doubting (and uncertain about) something, then leave it, for the heart of
the believer is at ease with the truth and is doubtful about lies and
falsehood. Being doubtful about something is an indication that it may be
false and wrong, so beware of it; and your feeling at ease with something is
an indication that it is true, so take hold of it.

End quote from Fayd al-Qadeer
3/529 

Shaykh Ibn ‘Uthaymeen (may Allah
have mercy on him) said: This hadeeth is an example of concise speech, and
how excellent and beneficial it is to follow it. One may feel doubtful about
many things, so we say: leave that which you have doubts about for that
which you have no doubts about, so that you may be safe and sound. If anything
makes you feel doubtful and anxious, then leave it for something that does
not make you feel doubtful, so long as this does not reach the level of
waswaas (when you start doubting everything). If it does reach the level of
waswaas, then do not pay any attention to it. 

This has to do with acts of
worship, interactions with others, marriage and all categories of
knowledge. 

An example of that in the case
of acts of worship is that of a man who breaks his wudoo’, then prays, and
he is not sure whether he did wudoo’ after breaking it or not. He is unsure,
because if he did do wudoo’ then his prayer is valid, but if he did not then
his prayer is invalid, so he remains anxious. In this case we say: Leave
that which makes you doubt for that which does not make you doubt. The doubt
in this case has to do with whether the prayer was valid and not doubting
means that you should do wudoo’ and pray. 

The opposite of the previous
example is that of a man who does wudoo’ then prays, and he is not sure
whether he broke his wudoo’ or not. 

In this case we say: Leave that
which makes you doubt for that which does not make you doubt. You have
something which is certain, which is that you did wudoo’, then you were not
sure whether this wudoo’ was broken or not. What you should ignore is the
doubt or uncertainty as to whether the wudoo’ was broken or not. So calm
down and ignore that doubt or uncertainty.

End quote from Sharh al-Arba‘een
an-Nawawiyyah, p. 155 

This hadeeth is a basic
principle in the area of being prudent and is encouragement to ignore
doubts. It is also a basic principle in the area of following that which is
certain and ignoring that which is dubious. 

Al-Haafiz Ibn Rajab (may Allah
have mercy on him) said:

This hadeeth highlights the
importance of being careful when faced with doubtful and dubious matters,
and keeping away from them, because in the case of that which is definitely
halaal, the believer will have no doubt in his heart concerning it – and
doubt here means anxiety and worry – rather he will feel at ease and will
have reassurance in his heart. As for that which is doubtful, he will feel
anxiety and worry concerning it.

End quote from Jaami‘ al-‘Uloom
wa’l-Hukam, 1/280 

And Allah knows best.