Judges 16:9

While the men were hidden in her room, she called out, “Samson, the Philistines are here!” But he snapped the bowstrings like a strand of yarn seared by a flame. So the source of his strength remained unknown.

Now there were men lying in wait, abiding with her in the chamber. And she said unto him, The Philistines be upon thee, Samson. And he brake the withs, as a thread of tow is broken when it toucheth the fire. So his strength was not known.