1 Samuel 17:55-58

(55) And when Saul saw David go forth against the Philistine, he said unto Abner, the captain of the host, Abner, whose son is this youth? And Abner said, As thy soul liveth, O king, I cannot tell. (56) And the king said, Enquire thou whose son the stripling is. (57) And as David returned from the slaughter of the Philistine, Abner took him, and brought him before Saul with the head of the Philistine in his hand. (58) And Saul said to him, Whose son art thou, thou young man? And David answered, I am the son of thy servant Jesse the Bethlehemite.

It should seem that Saul had forgotten the person of David, so as not to know him again, which would appear rather strange if it were not considered that his disease at that time had left him but little recollection; and his anxiety for his kingdom and his life, also now preyed, no doubt, much upon him. But when David made himself known to him, the matter must have been brought to his recollection.