法苑珠林 4.63

4.63

依起世經云。佛告諸比丘。或有外道來問汝云。何因緣故虛空中有是聲耶。汝應答云。有三因緣更相觸故。雲聚空中有音聲出。何者為三。一雲中風界與地界相觸著故。便有聲出。二於雲中風界與彼水界相觸著故。即便聲出。三於雲中風界與彼火界相觸著故。即便聲出。所以者何。譬如樹枝相揩即有火出。此亦如是（依經雷亦多種。或有雷車鼓鬼神桴打手擊。故俗云稱為天鼓於中亦有罪惡多者霹靂而死。見受報也）。

According to the Lokotpatti-sūtra: The Buddha told the monks, 'If non-Buddhists come and ask you, "By what cause and condition is there this sound in the sky?", you should reply, "By three causes and conditions striking against one another, sounds issue forth from the cloud-masses in the sky. What are the three? First, because the wind element within the clouds strikes and contacts the earth element, sound issues forth. Second, because the wind element within the clouds strikes and contacts that water element, sound issues forth. Third, because the wind element within the clouds strikes and contacts that fire element, sound issues forth. Why is this? Just as when tree branches rub against one another fire issues forth, this is also like that."' (According to the sūtras, thunder is of many kinds: there are thunder-chariots and thunder-drums that spirits and deities beat with mallets or strike with their hands; hence common speech calls them 'heavenly drums'. Among them, those who possess much evil and sin are struck dead by thunderbolts, visibly receiving their retribution.)