Rashi on Genesis 38:1/רש"י על בראשית ל״ח:א׳

1ויהי בעת ההוא AND IT CAME TO PASS AT THAT TIME — Why is this section placed here thus interrupting the section dealing with the history of Joseph? To teach that his brothers degraded him from his high position. When they saw their father’s grief they said, “You told us to sell him: if you had told us to send him back to his father we would also have obeyed you” (Genesis Rabbah 85:2).

2ויט AND HE TURNED away from his brothers.

3עד איש עדלמי UNTO A CERTAIN ADULLAMITE — he entered into a business-partnership with him.