瑜伽師地論略纂 15.164

15.164

論云「佛為首大苾芻僧」者，謂佛為首，言簡異外道。外道弟子亦言苾芻，自言我是大自在天為首苾芻，今言佛為首，故簡彼也。

Regarding the treatise's statement, "The great saṃgha of monks with the Buddha at its head": saying "with the Buddha at its head" excludes non-Buddhists. Disciples of non-Buddhists are also called bhikṣus, saying of themselves, 'We are bhikṣus with Maheśvara at our head.' Now saying "with the Buddha at its head" therefore excludes them.