Midrasch zu Schoftim 14:17

Two men welcomed two righteous men and were blessed because of them. At first they had no sons, but after these righteous men entered their homes and then departed, the Holy One, blessed be He, gave them sons. These two were Laban and Jethro. If Laban had any sons of his own, would his daughters have been forced to shepherd his flock? The fact is that he had no sons, but after Jacob’s arrival, he was blessed with property and sons, as it is written: And the Lord hath blessed me for thy sake (Gen. 30:27), and furthermore, it is written: And he heard the words of Laban’s sons (ibid. 31:1). In this instance, it is written: Now the priest of Midian had seven daughters (Exod. 2:16). Is it likely that if he had sons his daughters would have been shepherdesses? No. Obviously, he had no sons. After Moses entered his house, however, he was blessed with sons, as it is written: And the sons of the Kenite, Moses’s father-in law (Judg. 14:17).