Tosefta su Ecclesiaste 2:14

The halakhah always follows the words of Beth Hillel. One who wishes to be strict upon oneself and to follow the strictures of both Beth Hillel and Beth Shammai, regarding this person it is said “the fool walks in darkness” (Ecclesiastes 2:14), while one who accepts the leniencies of Beth Shamai and the leniencies of Beth Hillel is evil – [one must follow] either Beth Shammai in accordance with their leniencies and strictures, or Beth Hillel in accordance with their leniencies and strictures. Beit Hillel says, a man may not send out his wife with an old get, so her get will not be older than her child. If he wrote in the name of a provincial government or an office, or if there were two standing kings and he wrote it in the name of one of them, it is valid. If he wrote it in the name of his father’s father, it is valid. In the name of his family, it is invalid. But if he was called by the same name, it is valid.