It's in Bran 3 in ACOK :
"Something his father had told him once when he was little came back to him suddenly. He had asked Lord Eddard if the Kingsguard were truly the finest knights in the Seven Kingdoms. “No longer,” he answered, “but once they were a marvel, a shining lesson to the world.”
“Was there one who was best of all?”
“The finest knight I ever saw was Ser Arthur Dayne, who fought with a blade called Dawn, forged from the heart of a fallen star. They called him the Sword of the Morning, and he would have killed me but for Howland Reed.” Father had gotten sad then, and he would say no more. Bran wished he had asked him what he meant.