… “he should dwell contemplating suffering in form … as above … he is freed from suffering, I say.”
SN22.41: Tatiyaanudhammasutta
Tatiyaanudhammasutta: In Accordance with the Dhamma (3) - translated by Bhikkhu Bodhi
Translations:Thanissaro Bhikkhu
Translated by Bhikkhu Bodhi. Source text via SuttaCentral.