釋氏要覽 2.15

2.15

白虎通云。人之所以相拜者。何以表情見意屈節卑體尊事者也。拜之言服也。俗中兩拜者。蓋法陰陽也。今釋氏以三拜首。蓋表三業歸敬也。智論云。內式禮拜。大約身口業也。佛法以心為本。以身口為末。故三拜為禮數也。

The Baihu tong states: Why do people bow to one another? It is to express sentiments and show intention, bending their status and humbling their bodies to revere their superiors. "Bowing" [bai] means submission. In secular custom, two bows model yin and yang. Now, the Buddhists take three prostrations as primary, representing the devotion and reverence of the three karmas. The [Dazhi]du lun states: The internal protocol of prostration generally pertains to the karma of body and speech. The Buddha-Dharma takes mind as root and body and speech as branches; therefore, three prostrations serve as the measure of etiquette.