Yoga Rahasya Chapter One verse 33

abhyāsa samaye nityaṃ recapūraka kumbhakān |
yathā śakti prakuḥ vīta rītya ujyāyvā vilambataḥ ||

During the practice of Āsana one must constantly engage
in regulating the exhale, inhale and retention.
Ujjāyī is to be smooth and slow,
according to the strength or capability of the student.

abhyāsa - practicesamaya - circumstances; appointed or proper time; agreement; regulatenitya - eternal; constantly dwelling or engaged inrecaka - exhalepūraka - inhalekumbhaka - retentionyathā - according asśakti - power, energy; ability, capability; strength; might, effort;vīta - free from; smoothujjāyī - upwards victoryvilamba - slowness