I Samuel 3:8-9

And the LORD called Samuel again the third time. So he arose and went to Eli, and said, “Here I am, for you did call me.” Then Eli perceived that the LORD had called the boy. Therefore Eli said to Samuel, “Go, lie down; and it shall be, if He calls you, that you must say, ‘Speak, LORD, for Your servant hears.’ ” So Samuel went and lay down in his place.