摩訶止觀 4.99

4.99

頭陀乞食者，行人不能即事而中修實相慧者，當次第三觀調心而入中道。次第觀故，名為「乞食」；亦見中道，又名飽義。即中士也。

As for begging food through dhūta: when a practitioner cannot directly realize the middle within phenomena and cultivate the wisdom of true reality, they should regulate the mind through the sequential three contemplations to enter the middle way. Because of sequential contemplation, it is called 'begging food'; also seeing the middle way, it is again called the meaning of being filled. These are the intermediate persons.