Midrash sobre Zevahim 3:8

(The brethren) sold him to the Ishmaelites for twenty pieces of silver, and each one of them took two pieces of silver (apiece) to purchase shoes for their feet, as it is said, "Thus saith the Lord,… Because they have sold the righteous for silver, and the needy for a pair of shoes" (Amos 2:6). They said: Let us swear among ourselves that no one of us shall declare the matter to our father Jacob. Judah said to them: Reuben is not here, and the ban cannot be valid through nine (adults). What did they do? They associated the Omnipresent with them and proclaimed the ban.