I Samuel 3:7-11

7
Now Samuel did not yet know the LORD: The word of the LORD had not yet been revealed to him.

8
A third time the LORD called, “Samuel!” And Samuel got up and went to Eli and said, “Here I am; you called me.” Then Eli realized that the LORD was calling the boy.

9
So Eli told Samuel, “Go and lie down, and if he calls you, say, ‘Speak, LORD, for your servant is listening.’” So Samuel went and lay down in his place.

10
The LORD came and stood there, calling as at the other times, “Samuel! Samuel!” Then Samuel said, “Speak, for your servant is listening.”

11
And the LORD said to Samuel: “See, I am about to do something in Israel that will make the ears of everyone who hears about it tingle.

- biblegateway.com