佛祖統紀 3.41

3.41

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.