1 Kings 3:9-10

And said: Here am I; for thou didst call me. Then Heli understood that the Lord called the child, and he said to Samuel: Go, and sleep. And if he shall call thee any more, thou shalt say: Speak, Lord, for thy servant heareth. So Samuel went and slept in his place. And the Lord came and stood. And he called, as he had called the other times: Samuel, Samuel. And Samuel said: Speak, Lord, for thy servant heareth.