Then Eli understood that the LORD was calling the youth.
So Eli said to Samuel, “Go to sleep, and if you are called, reply,
‘Speak, LORD, for your servant is listening.’”  (1 Sam 3:1-10, 19-20)
https://bible.usccb.org/bible/readings/011222.cfm
The Call of Samuel is one of my favorite Bible stories. I remember hearing this story when I wasn’t much older than Samuel. I always thought that Samuel was a lucky kid to be sleeping near the Ark of the Covenant and to hear God calling. But, of course, it was the old priest Eli who realized that it was God calling the child. And so Samuel answers God, “Speak, Lord, for your servant is listening.”