Targum zu Bereschit 46:21

The sons of Benjamin, ten; and their names, according to the interpretation of Joseph his brother: Bela, who was swallowed up from him; Beker, who was the chosen of his mother; Eshbal, who went into captivity; Gera, who became a sojourner in a foreign land; Naaman, who was pleasant and honourable; Achi, who had a (twin) brother, the son of his mother; Rosh, who was a chief in his father's house; Muppim, who was sold into Muph; Chuppim, because at the time that he was separated from him he was the son of eighteen years and was eligible for marriage (Chupha, thalamus nuptialis); and Ared, who descended into Mizraim.