1 Samuel 12:18
So Samuel called unto the LORD; and the LORD sent thunder and rain that day: and all the people greatly feared the LORD and Samuel. So Samuel called unto the LORD; and the LORD sent thunder and rain that day: and all the people greatly feared the LORD and Samuel.
The setting is Gilgal, where Samuel gave his parting words as ruler of Israel after the people had welcomed Saul as king. The thunder and rain fell on wheat-harvest day, in May and June when weather in the East was normally dry, so such weather was unexpected. Samuel called unto the Lord in prayer, and the Lord answered that same day by sending thunder and rain. It was a sudden, violent storm sent in response to his prayer.
The storm was a sign from God of His displeasure, showing that Israel's demand for a king was a great sin that added to their sins. It showed that in demanding a king they rejected God's rule and authority over them and His power over them. It confirmed Samuel's warning and showed that God had honored the man the people had rejected. All the people were terrified and greatly feared the Lord and Samuel.
They acknowledged and confessed that asking for a king had added to their sins, and asked Samuel to pray for them. Israel had wanted a human ruler instead of being satisfied with God as their ruler, and although they could have withdrawn their demand, pride kept them from doing so. Rejecting Samuel's leadership was in fact dishonoring and rejecting the Lord who had ruled them, not merely rejecting Samuel.