Majjhima Nikaya 3:63
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.’