1 Samuel 3:9-10

Therefore Eli said to Samuel, “Go, lie down. It shall be, if he calls you, that you shall say, ‘Speak, Yahweh; for your servant hears.’” So Samuel went and lay down in his place. Yahweh came, and stood, and called as at other times, “Samuel! Samuel!” Then Samuel said, “Speak; for your servant hears.”