佛說盂蘭盆經疏 2.80

2.80

When these words were spoken, it was also after the offering had been set out; it did not happen on a single occasion. It was only when they reached the Pippala Cave that it was first compiled into a sūtra. Maudgalyāyana cared for his own parents and extended it to others, just as Ying Kaoshu admonished Duke Zhuang.