Trung Bộ Kinh (Majjhima Nikaya) 3:74
SUJATO‘nirodhānupassī assasissāmī’ti sikkhati, ‘nirodhānupassī passasissāmī’ti sikkhati;
They practice like this: ‘I’ll breathe in observing cessation.’ They practice like this: ‘I’ll breathe out observing cessation.’