1 Samuel 21:11
And the servants of Achish said unto him, Is not this David the king of the land? did they not sing one to another of him in dances, saying, Saul hath slain his thousands, and David his ten thousands? And the servants of Achish said unto him, Is not this David the king of the land? did they not sing one to another of him in dances, saying, Saul hath slain his thousands, and David his ten thousands?
David was away from his own people, seeking refuge in Gath, when Achish's servants spoke about him. They recognized him as the king of the land, showing how famous he already was in Israel. They knew him by the celebratory song that set Saul's thousands slain against David's tens of thousands. That song was remembered as celebrating his victories over the Philistines, including Goliath.
Their recognition put him in great danger while he was seeking refuge. He was shut up and acted as though he were insane, feigning madness in order to escape.