1 Samuel 3:6-9

6And the Lord called again, "Samuel!" and Samuel arose and went to Eli and said, "Here I am, for you called me." But he said, "I did not call, my son; lie down again."

7Now Samuel did not yet know the Lord, and the word of the Lord had not yet been revealed to him.

8And the Lord called Samuel again the third time. And he arose and went to Eli and said, "Here I am, for you called me." Then Eli perceived that the Lord was calling the boy.

9Therefore Eli said to Samuel, "Go, lie down, and if he calls you, you shall say, 'Speak, Lord, for your servant hears.'" So Samuel went and lay down in his place.