Advaitins believe that the world is mithya. There are advaita commentaries on the brahmasutras, most notably by Shankara himself. So according to advaitins, which brahmasutra or sutras say that the world is mithya?
EDIT: Replying to comments. According to the advaita school, the brahmasutras DO teach advaita alone. So it is reasonable to ask which brahmasutra teaches that the world is mithya. Secondly, a question about where brahmasutras teach that the world is satya, is irrelevant here. If someone wants to know this, they should post it as a separate question.