So Samuel arose and went to Eli and said, "Here I am, for you called me." Eli answered, saying, "I did not call you, my son; lie down again." ... 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, your must say, 'Speak, Lord, for your servant is listening.'" So Samuel went and lay down in his place.
1 Samuel 3:6-9