Mishnà su Levitico 18:18

[In the case of] three brothers, of whom two were married to two sisters and one was unmarried, if one of the husbands of the sisters died and the unmarried one betrothed her (maamar), and afterwards his other brother died, Beth Shammai says: his wife remains with him, and the other [widow] is released on the grounds of [the law forbidding] the wife’s sister. But Beth Hillel says: he should put away his wife with a get and halitzah, and the wife of his brother [he should put away] with halitzah. This is the case of which they said: woe to him because of his wife, and woe to him because of his brother’s wife!