Kāyasutta

sn46.2:15.1 And what fuels the arising of the awakening factor of equanimity, or, when it has arisen, fully develops it?

sn46.2:15.2 There are things that are grounds for the awakening factor of equanimity.

sn46.2:15.3 Frequent rational application of mind to them

sn46.2:15.4 fuels the arising of the awakening factor of equanimity, or, when it has arisen, fully develops it.