Majjhima Nikaya 2:21
SUJATO‘Passambhayaṁ kāyasaṅkhāraṁ assasissāmī’ti sikkhati, ‘passambhayaṁ kāyasaṅkhāraṁ passasissāmī’ti sikkhati.
They practice like this: ‘I’ll breathe in stilling the physical process.’ They practice like this: ‘I’ll breathe out stilling the physical process.’