Midrasch zu Bereschit 8:2

... Our teachers have said: Once, while Moses our Teacher was tending [his father-in-law] Yitro’s sheep, one of the sheep ran away. Moses ran after it until it reached a small, shaded place. There, the lamb came across a pool and began to drink. As Moses approached the lamb, he said, “I did not know you ran away because you were thirsty. You are so exhausted!” He then put the lamb on his shoulders and carried him back. The Holy One said, “Since you tend the sheep of human beings with such overwhelming love - by your life, I swear you shall be the shepherd of My sheep, Israel.”