Yoga Rahasya Chapter One verse 33

abhyāsa-samaye nityaṃ reca-pūraka-kumbhakān |
yathā-śakti prakuḥ vīta rītya-ujyāyvā vilambataḥ ||

During the practice 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 - practice; the effort of the mind to remain in its unmodified condition of purity ; repeated or permanent exercise, discipline, use, habit, custom;samaya - 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