一切經音義 2.186

2.186

虛空（許居反說文從虍從丘或作虛並古文丘字也虍音呼經作虗不成字也）。

虛空 (Pronounced 許 + 居 [*xū*]. The *Shuowen* says: 'Takes the semantic classifier 虍 and 丘; sometimes written 虛, both being ancient forms of the character 丘.' 虍 is pronounced 呼 [*hū*]. The sūtra text writes it as 虗, which does not form an authentic character).