Majjhima Nikaya 2:31
SUJATOsamudayadhammānupassī vā kāyasmiṁ viharati, vayadhammānupassī vā kāyasmiṁ viharati, samudayavayadhammānupassī vā kāyasmiṁ viharati.
They meditate observing the body as liable to originate, as liable to fall, and as liable to both originate and vanish.