Genesis 45:26 – They said to him [Yaacov] saying “Yosef is still alive”, and he is the ruler over all the land of Egypt; but he had a turn of heart, for he did not believe them.
Avot D’Rabbi Natan (Chapter 30) – Rabbi Shimon said “this is the punishment of liars that even when they say the truth they are not believed.” This is what we find by the sons of Yaacov as it says “He recognized the garment and said this is the cloak of my son”. That is why afterwards even though they finally told him the truth, he didn’t believe them as it says “he had a turn of heart, for he did not believe them.”
Torah Temimah Colloquial Translation on Note #12:
Even though in the end the brothers were believed, it was because of the signs [from Yosef] that they showed him such as the wagons that Pharoah had sent. Also, as is mentioned in the Midrash that they showed him the sign that indicated what Yosef and Yaacov had been studying together when he was taken away.
DBS Note: I wonder if the Torah Temimah went out of his way to quote Avot D’Rabbi Natan because of the important ethical lesson here.