This is a serious matter that scholars have addressed in detail. The majority of scholars, including the Hanafi, Shafi'i, and Hanbali schools, hold that the Hajj technically fulfills the obligation (i.e., the person is not required to repeat it), but the person earns no reward and carries the sin of using haram wealth in worship. The Maliki school, however, holds that the Hajj is not valid if funded entirely with haram money, because the ability (istitaa'ah) to perform Hajj must come from lawful means.
Sheikh Ibn Baz stated that a Muslim must ensure their Hajj is funded from lawful (halal) earnings. The Prophet (peace be upon him) said that Allah is good and accepts only that which is good. Using interest money, stolen funds, or income from prohibited activities to fund Hajj contradicts the very essence of worship, which is submission to Allah through lawful means. He strongly warned against performing Hajj with haram money and advised that the person should first purify their income.
Sheikh Al-Albani emphasized that 'Hajj mabrur' (an accepted, virtuous Hajj) has no reward other than Paradise, but achieving Hajj mabrur requires lawful spending, good character during the pilgrimage, and following the Sunnah. A person who performs Hajj with stolen or interest-based money may have technically discharged the obligation (he need not repeat it), but has deprived himself of the immense reward of Hajj mabrur. The person must repent, return any stolen wealth to its rightful owners, and ensure future acts of worship are funded from halal income.