פירוש על קידושין 57:18

According to the rabbis, if there is not enough money to redeem both him and his son, his primary responsibility is to redeem himself. This is like putting the oxygen mask on yourself first on an airplane. But R. Judah says that the mitzvah to redeem lies primarily on the father. So first he must redeem his son, and only then, when he has the money, he must redeem himself.