Judges 3:12-30 tells Ehud's story. For your enjoyment and biblical education, I've included the entire passage from the ESV here:
And the people of Israel again did what was evil in the sight of the LORD, and the LORD strengthened Eglon the king of Moab against Israel, because they had done what was evil in the sight of the LORD. He gathered to himself the Ammonites and the Amalekites, and went and defeated Israel. And they took possession of the city of palms. And the people of Israel served Eglon the king of Moab eighteen years.
Then the people of Israel cried out to the LORD, and the LORD raised up for them a deliverer, Ehud, the son of Gera, the Benjaminite, a left-handed man. The people of Israel sent tribute by him to Eglon the king of Moab. And Ehud made for himself a sword with two edges, a cubit in length, and he bound it on his right thigh under his clothes.
And he presented the tribute to Eglon king of Moab. Now Eglon was a very fat man. And when Ehud had finished presenting the tribute, he sent away the people who carried the tribute. But he himself turned back at the idols near Gilgal and said, “I have a secret message for you, O king.” And he commanded, “Silence.” And all his attendants went out from his presence. And Ehud came to him as he was sitting alone in his cool roof chamber. And Ehud said, “I have a message from God for you.” And he arose from his seat. And Ehud reached with his left hand, took the sword from his right thigh, and thrust it into his belly. And the hilt also went in after the blade, and the fat closed over the blade, for he did not pull the sword out of his belly; and the dung came out. Then Ehud went out into the porch and closed the doors of the roof chamber behind him and locked them.
When he had gone, the servants came, and when they saw that the doors of the roof chamber were locked, they thought, “Surely he is relieving himself in the closet of the cool chamber.” And they waited till they were embarrassed. But when he still did not open the doors of the roof chamber, they took the key and opened them, and there lay their lord dead on the floor.
Ehud escaped while they delayed, and he passed beyond the idols and escaped to Seirah. When he arrived, he sounded the trumpet in the hill country of Ephraim. Then the people of Israel went down with him from the hill country, and he was their leader. And he said to them, “Follow after me, for the LORD has given your enemies the Moabites into your hand.” So they went down after him and seized the fords of the Jordan against the Moabites and did not allow anyone to pass over. And they killed at that time about 10,000 of the Moabites, all strong, able-bodied men; not a man escaped. So Moab was subdued that day under the hand of Israel. And the land had rest for eighty years. (Judges 3:12-30, ESV)