Godattasutta

sn41.7:4.1 And what is the release of the heart through emptiness?

sn41.7:4.2 It’s when a mendicant has gone to a wilderness, or to the root of a tree, or to an empty hut, and reflects like this:

sn41.7:4.3 ‘This is empty of a self or what belongs to a self.’

sn41.7:4.4 This is called the release of the heart through emptiness.