妙法蓮華經文句 2.90

2.90

觀心釋者，三智名觀，三諦名世，三觀是語，本故名音。

Explanation by contemplation of mind: the three wisdoms are called 'observing' (guan); the three truths are called the 'world' (shi); the three contemplations are speech, and being their root, are called 'sound' (yin).