Shaykh Abdullaah Al-Ghudayyaan said, “This [informing the police in this case] is an obligation.
It is mandatory that he informs them [the police] because this is transgression without any right; this is oppression on his part.
The Prophet sallallaahu ‘alaihi wa sallam said, “Aid your brother whether he is the oppressed or the oppressor.”
The Prophet sallallaahu ‘alaihi wa sallam was asked, “How do you aid an oppressor?” He said,“Prevent him from his oppression.” [Bukhari no. 2286]
[Informing the authorities] will prevent a Muslim from being unjust, and this will prevent people from being oppressed due to the result of [his plans]. From another angle, this will be beneficial to the society as there are many Muslims amongst them.
So this can be considered from [the following] three aspects:
1. Helping the one who intends to commit suicide [or some other forms of terrorist attack], [by preventing him from his oppression].
2. Helping those who will feel the consequences of this attack, due to the killing and injuries that would occur.
3. Helping the other Muslims, those who might be harmed by this.
These three aspects are including in the saying of the Prophet sallallaahu ‘alaihi wa sallam: “Aid your brother whether he is the oppressed or the oppressor.”
And all success lies with Allaah.” Ref:masjidbinbaz.co.uk
Posted by Abdul Kareem Ibn Ozzie