Genesis 38:11

Remain a widow till Shelah my son be grown The contract of marriage, it seems, was so understood, even before any positive law was made on the subject, that, if the husband died without any issue, his next brother was to marry his wife, and as long as any of his brethren remained they were bound to marry her, if left a widow. Accordingly Shelah, the third son, was reserved for Tamar, yet with design that he should not marry so young as his brothers had done. For it would seem from Judah's expression, Lest peradventure he die also, that he thought marrying too young was the cause of their death; though some consider his conduct as an evidence that he never intended to give his son to her.