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 Haji 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 Haji is not valid if funded entirely with haram money, because the ability (istitaa'ah) to perform Haji must come from lawful means.
Sheikh Ibn Baz stated that a Muslim must ensure their Haji is funded from lawful (halal) earnings. Nabi (shallallahu alaihi wa sallam) said that Allah is good and accepts only that which is good. Using interest money, stolen funds, or income from prohibited activities to fund Haji contradicts the very essence of worship, which is submission to Allah through lawful means. He strongly warned against performing Haji with haram money and advised that the person should first purify their income.
Sheikh Al-Albani emphasized that 'Haji mabrur' (an accepted, virtuous Haji) has no reward other than Paradise, but achieving Haji mabrur requires lawful spending, good character during the ibadah haji, and following the Sunnah. A person who performs Haji 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 Haji mabrur. The person must repent, return any stolen wealth to its rightful owners, and ensure future acts of worship are funded from halal income.