佛祖統紀 3.41

3.41

初頓教者。即第一華嚴時。從部時味。得名為頓。此謂如來始成正覺。在寂滅道場。四十一位法身大士。及宿世根熟。天龍八部。一時圍遶。如雲籠月。是時如來現盧舍那身。說圓滿修多羅。故言頓教。

First, the Sudden Teaching: this is the first period, the Huayan period. From its section, period, and flavor, it gets the name 'Sudden'. This refers to when the Tathāgata first attained sambodhi in the bodhimaṇḍa of extinction, where the Mahāsattvas of the forty-one stages of the Dharmakāya, as well as the devas, nāgas, and eight classes whose capacities had matured in past lives, gathered in a circle all at once, like clouds enclosing the moon. At that time, the Tathāgata manifested the body of Rocana and expounded the complete sūtras; hence it is called the Sudden Teaching.