Adam Clarke on Psalms 35:13

When they were sick - This might refer to the case of Absalom, who was much beloved of his father, and for whose life and prosperity he no doubt often prayed, wept, and fasted.

My prayer returned into mine own bosom - Though from the wayward and profligate life they led, they did not profit by my prayers, yet God did not permit me to pray in vain. They were like alms given to the miserable for God's sake, who takes care to return to the merciful man tenfold into his bosom. The bosom is not only the place where the Asiatics carry their purses, but also where they carry any thing that is given to them.