The story is described in this chapter of the Srimad Bhagavatam. Most people only know about how Vishnu's incarnation Vamana the dwarf took two steps, and then told Mahabali that there was no room for the third step, whereupon Mahabali offered his own head for the third step. What is not as well-known, however, is that Vishnu was so pleased at Mahabali's devotion and honesty that he granted him a boon that he would become the Indra of the next Manvantara. Here's what Vishnu says:
Bali Mahārāja has become the most famous among the demons and nonbelievers, for in spite of being bereft of all material opulences, he is fixed in his devotional service. Although bereft of his riches, fallen from his original position, defeated and arrested by his enemies, rebuked and deserted by his relatives and friends, although suffering the pain of being bound and although rebuked and cursed by his spiritual master, Bali Mahārāja, being fixed in his vow, did not give up his truthfulness. It was certainly with pretension that I spoke about religious principles, but he did not give up religious principles, for he is true to his word. Because of his great tolerance, I have given him a place not obtainable even by the demigods. He will become King of the heavenly planets during the period of the Manu known as Sāvarṇi. Until Bali Mahārāja achieves the position of King of heaven, he shall live on the planet Sutala, which was made by Viśvakarmā according to My order. Because it is especially protected by Me, it is free from mental and bodily miseries, fatigue, dizziness, defeat and all other disturbances.
Bali Mahārāja, you may now go live there peacefully. O Bali Mahārāja [Indrasena], now you may go to the planet Sutala, which is desired even by the demigods. Live there peacefully, surrounded by your friends and relatives. All good fortune unto you. On the planet Sutala, not even the predominating deities of other planets, what to speak of ordinary people, will be able to conquer you. As far as the demons are concerned, if they transgress your rule, My disc will kill them. O great hero, I shall always be with you and give you protection in all respects along with your associates and paraphernalia. Moreover, you will always be able to see Me there. Because there you will see My supreme prowess, your materialistic ideas and anxieties that have arisen from your association with the demons and Dānavas will immediately be vanquished.
And true to his word, from then on Vamana became Mahabali's personal bodyguard in his palace at Sutalaloka, as described in numerous scriptures including the Vamana Purana and the Uttara Kanda of the Ramayana.