Bereishis Rabah (78:8) #1: This refers to the most merciful of the Avos - Avraham, who said "Chalilah Lech me'Asos ka'Davar ha'Zeh" (Bereishis 18:25; he prayed to save the Resha'im of Sedom.)

Bereishis Rabah (78:8) #2: This refers to the most merciful of the Avos - Yakov, who put his sons in back of his wives (he had more mercy on his sons than on his wives), and he passed in front of them. He said, it is better that [Esav] strike me, and not them. Radak - there is no greater mercy than a father's mercy on his son.

Malbim: This mercy comes due to natural closeness that he has to his son. There are two reasons. (a) A father knows that their nature is to be wayward, and he judges them favorably - so Hashem has mercy on those who fear Him, for the Nefesh is a part of Hashem, and He is close to it. (b) He knows that the body impedes it from going in Hashem's ways.