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, Kartaviryarjuna 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.