The Buddha tells Rāhula that a monk should cultivate the thought that, in the four elements, either in one’s own body or in external objects, there is neither self nor what pertains to the self. A.ii.164; this same topic is discussed in greater detail in the Ambalaṭṭhika-Rāhulovāda Sutta.

Buddhaghosa says (AA.ii.547) that the Buddha here declares emptiness in four things (catukotikasuññatā) — i.e., the four elements.