Trung Bộ Kinh (Majjhima Nikaya) 3:65
SUJATO‘sukhapaṭisaṁvedī assasissāmī’ti sikkhati, ‘sukhapaṭisaṁvedī passasissāmī’ti sikkhati;
They practice like this: ‘I’ll breathe in experiencing bliss.’ They practice like this: ‘I’ll breathe out experiencing bliss.’