法苑珠林 32.10

32.10

又處處經云。爾時佛笑。口中有五色光出者有五因緣。一欲令人有所問因所問有益故。二恐人言佛不知笑故。三為現口中光故。四笑諸不至誠故。五笑阿羅漢守空不得菩薩道。光還從頂上入者。當示後人大明故。

Furthermore, the Chuchu jing states: "When the Buddha smiled at that time, five-coloured light issued from his mouth for five reasons: first, because he wished to prompt someone to ask a question, and the question would be beneficial; second, for fear that people might say the Buddha does not know how to smile; third, in order to display the light within his mouth; fourth, to smile at those who lack sincere earnestness; fifth, to smile at the arhats who cling to emptiness and do not attain the bodhisattva path. That the light returned and entered through the crown of his head was to display great illumination to people of the future."