Majjhima Nikaya 3:99
SUJATOYasmiṁ samaye, bhikkhave, bhikkhu ‘aniccānupassī assasissāmī’ti sikkhati, ‘aniccānupassī passasissāmī’ti sikkhati;
Whenever a mendicant practices breathing while observing impermanence,
Yasmiṁ samaye, bhikkhave, bhikkhu ‘aniccānupassī assasissāmī’ti sikkhati, ‘aniccānupassī passasissāmī’ti sikkhati;
Whenever a mendicant practices breathing while observing impermanence,