The Buddha and Rāhula are on their way to the village for alms, and the Buddha tells Rāhula that all form (rūpa) should be regarded as not-self (anattā), and not only form, but also the other aggregates (khandhā). Rāhula stops and sits under a tree meditating. Sāriputta approaches and suggests that he should develop mindfulness of respiration (ānāpānasati). Later in the evening Rāhula asks the Buddha how he can do this. The Buddha describes how it is done by regarding all the elements — earth, water, fire, air, and space, both personal and external — with disgust and loathing of heart. One should not allow sensory impressions to lay hold of one’s heart, just as the earth remains impassive whatever may be thrown upon it. It is so with the other elements. One should grow in loving-kindness, compassion, sympathetic-joy, equanimity, contemplation of foulness, perception of impermanence, and in the mindfulness that comes from ordered breathing.
M.i.420‑6; it is perhaps a part of this sutta, which is quoted at Mil. 385, 388; see Mil. Trs.ii.312, n.1.