Patisambhidamagga

Anapanasati is probably the most popular practice in Buddhist meditation. Some watch the breath at the nostrils, some in the chest, some at the abdomen or some the whole breath. While none of them is expressly stated in the suttas, and all of them may be applicable, watching the breath at the tip of the nose has the unique position of being the practice expressly described in a text in the Tipitaka - the Patisambhidamagga.

The Patisambhidamagga is a Buddhist scripture, part of the Pali Canon of Theravada Buddhism. It is included there as the twelfth book of the Sutta Pitaka's Khuddaka Nikaya. Tradition ascribes it to the Buddha's disciple Sariputta. The text was dated by A.K. Warder to approximately 3rd century BCE.

"As the tree trunk placed on the level piece of ground, so the sign for the anchoring of mindfulness [at the nose tip or on the upper lip]. As the saw's teeth, so the in-breaths and out-breaths. As the man's mindfulness, established by the saw's teeth where they touch the tree trunk, without his giving attention to the saw's teeth as they approach and recede, though they are not unrecognized by him as they do so, and so he manifests endeavour, carries out a task and achieves a distinctive effect, so too the bhikkhu sits, having established mindfulness at the nose tip [where the breath touches (i.e. is felt) if breathing through the nose] or on the upper lip [where the breath touches (i.e. is felt) if breathing through the mouth], without giving attention to the in-breaths and out-breaths as they approach and recede, though they are not unrecognized by him as they do so, and he manifests endeavour, carries out a task and achieves a distinctive effect."