Ruling on telling the authorities about one who is stealing

It is proven in Saheeh Muslim (49) that Abu Sa’eed
al-Khudri (may Allaah be pleased with him) said: I heard the Messenger of
Allaah (peace and blessings of Allaah be upon him) say: “Whoever among
you sees an evil action, let him change it with his hand (by taking action);
if he cannot, then with his tongue (by speaking out); and if he cannot, then
with his heart (by hating it and feeling it is wrong), and that is the
weakest of faith.” 

Al-Nawawi (may Allaah have mercy on him) said: 

With regard to the words of the Prophet (peace and
blessings of Allaah be upon him) “let him change it”, this is a command in
the sense that it is obligatory according to the consensus of the ummah.
Enjoining what is good and forbidding what is evil is an obligation
according to Qur’aan and Sunnah and the consensus of the ummah, and it is
also part of the sincerity (naseehah) that is the faith. End quote. 

Sharh Saheeh Muslim (2/22) 

The Children of Israel would see evil but keep silent and not
denounce it, and that silence caused them to be cursed, as Allaah, may He be
glorified, says (interpretation of the meaning):

“Those among the Children of Israel who disbelieved were
cursed by the tongue of Dawood (David) and ‘Eesa (Jesus), son of Maryam
(Mary). That was because they disobeyed (Allaah and the Messengers) and were
ever transgressing beyond bounds.

79. They used not to forbid one another from Al-Munkar
(wrong, evildoing, sins, polytheism, disbelief) which they committed. Vile
indeed was what they used to do”

[al-Maa’idah 5:78-79] 

Based on that, it is not permissible for the one who sees an
evil to keep quiet about it when he is able to denounce it and change it. He
should start with changing it with his hand (by taking action) if he is able
to do that, such as if he is responsible for the one who is doing the evil
and has some authority over him. If he cannot change it with his hand, then
he should denounce it with his tongue (by speaking out), by enjoining what
is good and forbidding what is evil, and by advising in the way that is
best. Denouncing it verbally also includes telling those who are able to put
a stop to this evil, and he must try hard to do that. If he is unable to do
that, then he can avoid sin by denouncing it in his heart. If he falls short
in denouncing the evil in the way that he is able to, then he will be a
partner in sin with the sinner. 

If he has tried his best and done his duty of enjoining what
is good and forbidding what is evil, and the sinner does not respond, there
is no sin on him, because Allaah says (interpretation of the meaning): 

“O you who believe! Take care of your ownselves. If you
follow the (right) guidance [and enjoin what is right (Islamic Monotheism
and all that Islam orders one to do) and forbid what is wrong (polytheism,
disbelief and all that Islam has forbidden)] no hurt can come to you from
those who are in error. The return of you all is to Allaah, then He will
inform you about (all) that which you used to do”

[al-Maa’idah 5:103].

Shaykh al-Shanqeeti (may Allaah have mercy on him) said: 

The one who denounces evil according to his ability is safe
from this sin, but the one who approves of it and keeps quiet about it is a
sinner like the one who does it. End quote. 

Adwa’ al-Bayaan (1/467). 

You have to advise these people, first of all, to fear Allaah
and to refrain from stealing the property of others, and they have to return
what they took previously. If they refrain, then you will have done the
right thing towards them and the owner of the property. But if they persist
in what they are doing, then you must inform those who can put a stop to
what they are doing, even if you tell the owner of the property himself. It
is not essential for you to make it known that you are the one who is
telling on them, rather it is sufficient to send a letter – for example – to
the owner of the property and tell him what is really going on. It is
preferable for it to be accompanied by documents that prove that what you
are saying is true, so that he will take the matter seriously. Thus you will
have fulfilled your duty and saved yourself from sin.