John Gill Commentary Psalms 35:13

"But as for me, when they were sick, my clothing was sackcloth: I afflicted my soul with fasting; And my prayer returned into mine own bosom." — Psalms 35:13 (ASV)

But as for me, when they were sick
Or under any disorder or distress of body or mind, when any misfortune or infirmity attended them; meaning Saul and his courtiers, before David was persecuted by them;

my clothing [was] sackcloth ;
that is, he was grieved, and mourned for them, it being usual to put on sackcloth in time of mourning; see (Genesis 37:34) ;

I humbled my soul with fasting ;
on the account of them, giving up himself to prayer for them, as follows:

and my prayer returned into mine own bosom ;
that is, he prayed privately and heartily for them, as for himself; he was constant in it, his heart was in it, and he took delight in it, and he was heard and answered; unless the sense should be, that his prayer was slighted by them, and so returned back to himself, as a present despised is returned; but however it was not without its effect, the good for which he prayed for them was returned by the Lord unto him.