This was said by the Blessed One, said by the Arahant, as I have heard:
“Bhikkhus, 1) dwell |contemplating the unattractive::seeing the disagreeable [asubhānupassī]| nature of the body. 2) Let mindfulness while breathing in and out be well established internally as first priority. 3) Dwell |observing impermanence::watching instability [aniccānupassī]| in all |conditioned phenomena::fabrications, concoctions, intentional constructs, karmic activity [saṅkhārā]|.
For those contemplating the unattractive nature of the body, the |underlying tendency towards desire::latent disposition towards sensuality [rāgānusaya]| regarding the element of |beauty::attractiveness [subha]| is abandoned. When mindfulness while breathing in and out is well established internally as the first priority, then |afflictive::troublesome [vighātapakkhika]| outward-directed discursive thinking tendencies do not arise. For those dwelling observing impermanence in all conditioned phenomena, |ignorance::fundamental unawareness or misunderstanding of the true nature of reality, not experientially understanding the four noble truths [avijjā]| is abandoned and |true knowledge::wisdom, the direct realization of truth [vijjā]| arises.”
The Blessed One spoke on this matter. In this regard, it is said:
“Contemplating the unattractive nature of the body,
mindful while breathing in and out;
With continuous effort, seeing clearly,
the calming down of all |intentional constructs::intentions, volitions, and choices expressed as mental, verbal, and bodily activities; thought formations and constructed experiences (including proliferative tendencies); processes that produce kamma [saṅkhāra]|.
Such a bhikkhu who sees rightly,
is therein released;
Who has attained perfect understanding, peaceful,
he is truly a sage gone beyond bonds.”
This matter too was spoken by the Blessed One, as I have heard.