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.

tasmin - in thissati - beingśvāsa - inhalationpraśvāsā - exhalationgati - motionviccheda - dividingprāṇāyāma - extending the breath

Commentaries and Reflections

TKV 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.