There is a popular belief that the Kartavirya Arjuna had thousand arms due to the blessing of Lord Dattatreya. Kartaviryarjuna wanted to obtain supernatural powers. He took the advice of Sage Garga for getting his desire fulfilled.
Sage Garga asked Kartavirya Arjuna to offer prayers to Lord Dattatreya, who was an incarnation of the Trimurti.
Therefore, Kartavlryarjuna and his wife went Narmada River and took bath in the river and did penance to please Dattatreya.
Lord Dattatreya was in intense meditation in a nearby place. Pleased with the penance of the king, Dattatreya asked him what boon he wanted.
Kartaviryarjuna asked for thousand arms. Dattatreya granted his wishes.
This story is found in the Brahma Purana.