Midrasch zu Schoftim 4:17

And Moses went and returned to Jethro, his father-in-law (Exod. 4:18). Scripture says elsewhere in reference to this verse: A friend loveth at all times, and a brother is born for adversity (Prov. 17:17). Who was this friend? It was Jethro, who welcomed Moses after he fled from Pharaoh. From this you learn that if anyone undertakes to perform a good deed (even if he does not do it), that good deed will never cease recurring in his home. Jethro welcomed into his home a redeemer who had fled from his enemy, and so from his house there descended one who welcomed an enemy who was fleeing from a redeemer and slew him. Who was this enemy? Sisera, as it is said: Howbeit Sisera fled away on his feet to the tent of Jael the wife of Heber the Kenite … then Jael took a tent pin, etc. (Judg. 4:17, 21). And it is also written: And the children of the Kenite, Moses’ father-in-law (ibid. 1:16). Hence it says: A brother is born for adversity (Prov. 17:17).