Trung Bộ Kinh (Majjhima Nikaya) 3:75
SUJATO‘paṭinissaggānupassī assasissāmī’ti sikkhati, ‘paṭinissaggānupassī passasissāmī’ti sikkhati.
They practice like this: ‘I’ll breathe in observing letting go.’ They practice like this: ‘I’ll breathe out observing letting go.’