Judges 6:39
And Gideon said unto God, Let not thine anger be hot against me, and I will speak but this once: let me prove, I pray thee, but this once with the fleece; let it now be dry only upon the fleece, and upon all the ground let there be dew. And Gideon said unto God, Let not thine anger be hot against me, and I will speak but this once: let me prove, I pray thee, but this once with the fleece; let it now be dry only upon the fleece, and upon all the ground let there be dew.
The verse belongs to the history in Judges 6-8, one of the repeated times when after backsliding and chastisement followed by confession, God raised up faithful and valiant men to put down idolatry and deliver the Israelites from their enemies, though when the deliverer died the people gradually returned to their idols. More specifically, it belongs to the setting of Gideon's call to deliver Israel from Midianite oppression, which followed Israel doing evil and lasted seven years until the people confessed their sins and God raised up a helper.
In the verse Gideon humbly asks God not to let anger burn against him for speaking once more, requesting a renewed sign in which only the fleece would be dry while dew covered all the ground. This second test reversed the first sign, in which the fleece had been wet while the ground around it was dry. By it Gideon sought further assurance that God had called him to lead Israel's deliverance. He asked again because doubt and unbelief suggested the first result was inconclusive, since wool naturally absorbs moisture from the air. God granted the request.
The request illustrates the practice of setting human-devised signs to decide duty, and relying on such tests is a delusion contrary to God's will that can lead to serious wrongdoing.