佛說盂蘭盆經疏 2.73

2.73

Since the pure karma was accomplished, he knew for certain that suffering would be left behind. Observing the cause confirms the fruit without the slightest discrepancy, like a sound and its echo; therefore he rejoiced and stopped weeping. It is like being imprisoned in worldly punishment: having entrusted oneself to a person of great power, once the wealth and bet are presented, one's heart is already joyful.