Маджхима Никая 2:86
SUJATOsamudayadhammānupassī vā vedanāsu viharati, vayadhammānupassī vā vedanāsu viharati, samudayavayadhammānupassī vā vedanāsu viharati.
They meditate observing feelings as liable to originate, as liable to fall, and as liable to both originate and vanish.