觀無量壽佛經疏 2.41

2.41

言世尊亦遣富樓那為王說法者。此明世尊慈悲意重，愍念王身忽遇囚勞，恐生憂悴。然富樓那者，於聖弟子中最能說法，善有方便開發人心。為此因緣，如來發遣為王說法，以除憂惱。

In saying 'the World-Honoured One also dispatched Pūrṇa to expound the Dharma for the king,' this clarifies that the World-Honoured One's compassionate intent was profound; he took pity on the king's person suddenly encountering imprisonment and feared that he would give rise to distress and exhaustion. Pūrṇa was the foremost among the sacred disciples in expounding the Dharma, skillful in expedient means to open the human mind. For this reason, the Tathāgata dispatched him to expound the Dharma for the king in order to eliminate distress and affliction.