MN 121: Cūḷasuññata Sutta (The Shorter Discourse on Emptiness)
The Buddha describes his own meditation on emptiness and tells Ānanda how a meditator can descend into emptiness herself through seclusion and wise attention.
For a more detailed, comparative analysis including a practice guide, see Bhikkhu Analayo’sarticle: “Gradual Entry into Emptiness”.