Hazrat Moulana Ashraf Ali Thaanwi (rahmatullahi ‘alaihi) mentioned:

Hazrat ‘Abdullah bin Mubaarak [rahmatullahi ‘alaihi] (a great luminary and ascetic, who was the student of Hazrat Imaam Abu Haneefah [rahmatullahi ‘alaihi]) says: “I prefer returning one dirham which is obtained through a doubtful source than giving 600 000 dirhams in charity.” Hazrat (rahmatullahi ‘alaihi) then mentioned that from this statement of Hazrat ‘Abdullah bin Mubaarak (rahmatullahi ‘alaihi) we understand the serious nature of doubtful wealth. It is extremely sad to find that nowadays let alone doubtful wealth, people do not even refrain from that wealth which is categorically declared haraam in sharee’ah. All that they are interested in is in increasing and accumulating their wealth, regardless of the manner or means through which the wealth is acquired. (Behishti Zewar Pg. 417)

Source: Ihyaauddeen.co.za