1 Samuel 3:8-9

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.