When the Philistines solved his riddle, Samson said they had plowed with his what?

Answer: Heifer

Samson replied, "If you had not plowed with my heifer, you would not have solved my riddle!" His wife had told them the answer.

“Before sunset on the seventh day, the men of the city said to Samson: “What is sweeter than honey? And what is stronger than a lion?” So he said to them: “If you had not plowed with my heifer, you would not have solved my riddle!””— Judges 14:18 (BSB)