1 Samuel 3:8-10

So the LORD called Samuel a third time. And he stood and went to Eli and said, “Here I am, for you did call me.” Then Eli understood that it was the LORD [who was] calling the boy. So Eli said to Samuel, “Go, lie down, and it shall be that if He calls you, you shall say, ‘Speak, LORD, for Your servant is listening.’ ” So Samuel went and lay down in his place. Then the LORD came and stood and called as at the previous times, “Samuel! Samuel!” Then Samuel answered, “Speak, for Your servant is listening.”