The Lord called Samuel the third time. Again Samuel got up and went to Eli and said, “Here I am. You called me.” Finally, Eli understood that the Lord was calling the boy.
Samuel did not yet know the Lord because the Lord had not spoken directly to him before.
Eli told Samuel, “Go to bed. If he calls you again, say, ‘Speak, Lord. I am your servant, and I am listening.’” So Samuel went back to bed.