John Gill Commentary Isaiah 38:5

"Go, and say to Hezekiah, Thus saith Jehovah, the God of David thy father, I have heard thy prayer, I have seen thy tears: behold, I will add unto thy days fifteen years." — Isaiah 38:5 (ASV)

Go and say to Hezekiah
Turn again, and tell him, (2 Kings 20:5) : thus says the Lord the God of David your father ;
this is said, to show that he remembered the covenant he made with David his father, concerning the kingdom, and the succession of his children in it; and that he had a regard to him, as walking in his steps:

I have heard your prayer ;
and therefore was not surely a foolish one, as Luther somewhere calls it, since it was heard and answered so quickly: I have seen your tears ;
which he shed in prayer, and so studiously concealed from others, when he turned his face to the wall:

behold, I will add unto your days fifteen years ;
that is, to the days he had lived already, and beyond which it was not probable, according to the nature of his disease, he could live; and besides, he had the sentence of death pronounced on him, and had it within himself, nor did he pray for his life; so that these fifteen years were over and above what he could or did expect to live; and because it was unusual in such a case, and after such a declaration made, that a man should live, and especially so long a time after, it is ushered in with a "behold", as a note of admiration; it being a thing unheard of, and unprecedented, and entirely the Lord's doing, and which, no doubt, was marvellous in the eyes of the king.