Unaware that Working in a Bank is Impermissible

As long as you did not know that this work was haram, and you left this job
as soon as you found out that it was haram, then you do not have to get rid
of this wealth, and there is nothing wrong with you benefiting from it,
because Allah says, after stating that riba (interest) is haram
(interpretation of the meaning):

“So whosoever receives an admonition from his Lord and stops consuming riba,
shall not be punished for the past.”

[2:275]

Al-Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: Among the
things that we learn from this verse is that whatever a person took of riba
before learning of the prohibition is permissible for him, on the condition
that he repents and stops (dealing with riba). End quote. 

(Tafseer
Soorat al-Baqarah: 377) 

The scholars of the Standing Committee for Issuing Fatwas said: 

With regard to the period during which you worked in the bank, we hope that
Allah will forgive you for that sin. With regard to the wealth that you
accumulated and received because of working in the bank during that period,
there is no sin on you for it, because you were unaware of its ruling. 

Al-Shaykh ‘Abd al-‘Azeez
ibn Baz, al-Shaykh ‘Abd al-Razzaq ‘Afeefi, al-Shaykh ‘Abd-Allah ibn Ghadyan,
al-Shaykh ‘Abd-Allah ibn Qa’ood.

Fatawa al-Lajnah al-Daimah (15/46). 

They also said: If the situation is as you describe, and you gave up working
there after being told that it is not permissible to work in a bank, then
there is no sin on you with regard to what you received from the bank in
return for working there for the number of months mentioned, and you do not
have to give it in charity; it is sufficient for you to repent from that,
may Allah forgive us and you, because Allah says (interpretation of the
meaning): “Allah has permitted trading and forbidden riba. So whosoever
receives an admonition from his Lord and stops consuming riba, shall not be
punished for the past; his case is for Allah (to judge)” [2:275]. End
quote. 

Al-Shaykh ‘Abd al-‘Azeez
ibn Baz, al-Shaykh ‘Abd al-Razzaq ‘Afeefi, al-Shaykh ‘Abd-Allah ibn Ghadyan,
al-Shaykh ‘Abd-Allah ibn Qa’ood.

Fatawa al-Lajnah al-Daimah (15/50, 51).

And Allah
knows best.