Yoga Sūtra Chapter Two verse 49
तस्मिन् सति श्वासप्रश्वास्योर्गतिविच्छेदः प्राणायामः ॥४९॥
tasmin-sati-śvāsa-praśvāsayoḥ gati-vicchedaḥ prāṇāyāmaḥ ||49||
Being in this,
dividing the motion of the inhalation and the exhalation
is extending the breath.
Commentaries and ReflectionsTKV Desikachar:
It is possible only after a reasonable mastery of Āsana Practice.
Prāṇāyāma leads to this.
Pratyāhāra, to see without the senses distracting or pulling the mind, and
Dhāraṇā, to see without the mind losing itself, because of colouring or expectations.
Dhyānam arises out of this.