Trung Bộ Kinh (Majjhima Nikaya) 3:69
SUJATO‘abhippamodayaṁ cittaṁ assasissāmī’ti sikkhati, ‘abhippamodayaṁ cittaṁ passasissāmī’ti sikkhati;
They practice like this: ‘I’ll breathe in gladdening the mind.’ They practice like this: ‘I’ll breathe out gladdening the mind.’