But the king spared Mephibosheth the son of Jonathan, the son of Saul, because of the Lord’s oath that was between David and Saul’s son Jonathan.

And David said, Is there still anyone left of the house of Saul to whom I may show kindness for Jonathan's sake? And of the house of Saul there was a servant whose name was Ziba. When they had called him to David, he said to him, Are you Ziba? He said, I, your servant, am he. The king said, Is there not still someone of the house of Saul to whom I may show the [unfailing, unsought, unlimited] mercy and kindness of God? Ziba replied, Jonathan has yet a son who is lame in his feet. read more.
And the king said, Where is he? Ziba replied, He is in the house of Machir son of Ammiel in Lo-debar. Then King David sent and brought him from the house of Machir son of Ammiel at Lo-debar. And Mephibosheth son of Jonathan, the son of Saul, came to David and fell on his face and did obeisance. David said, Mephibosheth! And he answered, Behold your servant! David said to him, Fear not, for I will surely show you kindness for Jonathan your father's sake, and will restore to you all the land of Saul your father [grandfather], and you shall eat at my table always.