Praano hy esha yah sarva-bhootair vibhaati
vijaanan vidvaan bhavate naativaadee
aatma-kreeda aatma-ratih kriyaavaan
esha brahma-vidaam varishthah

"Breath is the life of every human being. Knowing this, the man of realization does not talk anymore.
His breath is his work, his breath is his joy—that is a kriyavan. He is the greatest of the knowers of Brahman."

- Mundaka Upanishad 3:1:4

Tapah svaadhyayah eeshvarapranadhanani kriya yogah

"Kriya Yoga is breath control (tapas), study of the Self,
i.e. by remaining in the company of the soul (sva adhyaya),
and surrender to God (ishwara pranidhanani)"
- Patañjali's Yoga Sutras 2:1

Kriyaa yuktasya siddhih syaat, akriyasya katham bhavet
na shastra paanhamaatrena yogasiddhih prajaayate

"If you practice Kriya you will achieve success in life.
Without practice, no success.
Only by studying the scriptures you will not be a perfect one."
- Hatha Yoga Pradipika

Shaastraanyadheetyaapi bhavanti moorkhaa
yastu kriyaavaan purushah sa vidvaan
sucintitam caushadham aaturaanaam
na naama maatrena karotyarogam

"One who only reads scriptures is ignorant. One who practices Kriya is the one who knows. Reading the name of a medicine again and again will not cure the disease."
- Niti-sara-sangraha

The Blessed Lord spoke:
"I declared this indestructible science of yoga to Vivasvan. Vivasvan told it to Manu, and Manu taught it to Ikshwaku.
All those kings who were highly realized generation after generation were aware of this yoga. O Paramtapa (slayer of enemies, Arjuna)! Over a long period of time this secret yoga technique has declined in the world.
O Arjuna, you are my intimate friend, and you are my devotee, so today I have told you the very old secret and best transcendental mystery of brahma vidya, the science of the yoga of knowledge, which is truly profound."
- Bhagavad Gita 4:1-3

See also the Gheranda Samhita 1:8

See also the reference to the udan vayu (the fiftieth breath) in the Prashnopanishad 3:7