1 Samuel 3:7-9

Now Samuel did not yet know the LORD, neither was the word of the LORD yet revealed unto him.

And the LORD called Samuel again the third time. And he arose and went to Eli, and said, Here am I; for thou didst call me. And Eli perceived that the LORD had called the child.

Therefore Eli said unto Samuel, Go, lie down: and it shall be, if he call thee, that thou shalt say, Speak, LORD; for thy servant heareth. So Samuel went and lay down in his place.