Speak, Lord, your servant is listening.

1 Samuel 3: 1-10

"Speak. Lord, your servant is listening."

A young boy hears a voice and repeatedly goes to his mentor, the priest Eli. I didn't call you, son. Go back to bed. Finally the priest realizes what is happening and instructs the boy to answer God the next time he is called. Speak, Lord, your servant is listening. 

How do I know when you are speaking, Lord?