佛說處處經 1.20

1.20

"That when the Buddha smiles, five-colored light emerges from his mouth is due to five nidānas: first, he wishes to prompt people to ask questions, because following upon questions there is benefit; second, for fear that people might say the Buddha does not know how to smile; third, to manifest the light within his mouth; fourth, he smiles at all who are insincere; fifth, he smiles at the arhats who cling to emptiness and do not attain the bodhisattva path. That the light returns and enters through the crown of his head is to show the great brilliance to later people."