I Samuel 3:7-9

(Now Samuel did not yet know the LORD, nor was the word of the LORD yet revealed to him.) 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.