References – AN 7.67: The Simile of the Citadel 
“Is it accurate to describe mindfulness as a gatekeeper with four duties: 1) Recognize what’s entering the mind. 2) Discern if it should enter or not. 3) Allow it if it’s wholesome. 4) And prevent it if it’s not conducive to well-being and happiness?” Answered by Ajahn Karuṇadhammo. [Mindfulness] [Similes] [Discernment] [Right Effort] // [Right Mindfulness] [Mindfulness of body] [Mindfulness of feeling] [Mindfulness of mind] [Mindfulness of dhammas] [Clear comprehension] [Proliferation]
Sutta: AN 7.67.7: Gatekeeper simile.
Sutta: MN 10: Satipaṭṭhāna Sutta.
2021 Abhayagiri Online Thanksgiving Retreat, Session 1, Excerpt 2