Once again, for the third time, the LORD called to Samuel. He got up, went to Eli, and said, "Here I am, for you have called me." Then Eli realized that it was the LORD who was calling the boy.

1 Samuel 3:7
Now Samuel did not yet know the LORD, because the word of the LORD had not yet been revealed to him.

1 Samuel 3:9
"Go and lie down," he said to Samuel, "and if He calls you, say, 'Speak, LORD, for Your servant is listening.'" So Samuel went and lay down in his place.

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