Trung Bộ Kinh (Majjhima Nikaya) 3:66
SUJATO‘cittasaṅkhārapaṭisaṁvedī assasissāmī’ti sikkhati, ‘cittasaṅkhārapaṭisaṁvedī passasissāmī’ti sikkhati;
They practice like this: ‘I’ll breathe in experiencing the mental process.’ They practice like this: ‘I’ll breathe out experiencing the mental process.’