Unaware that Working in a Bank is Impermissible
Question

I did not know that working in a bank is haram (impermissible). I have repented and left this position. What is the ruling on the wealth I gained from working in a bank? Is it haram?
Praise be to Allah.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.