Study a single sutra by selecting the pada (chapter) and then the sutra (verse).
Source: English translation from The Yoga Sutras of Patanjali (Sri Swami Satchidananda)
That [firm posture] being acquired, the movements of inhalation and exhalation should be controlled. This is pranayama.
Source: English translation from Light on the Yoga Sutras of Patanjali (B.K.S. Iyengar)
Pranayama is the regulation of the incoming and outgoing flow of breath with retention. It is to be practised only after perfection in asana is attained.
Source: How to Know God: The Yoga Aphorisms of Patanjali (Swami Prabhavananda, Christopher Isherwood)
After mastering posture, one must practice control of the prana (pranayama) by stopping the motions of inhalation and exhalation.
Source: English translation from The Heart of Yoga (T.K.V. Desikachar)
Pranayama is the conscious, deliberate regulation of the breath replacing unconscious patterns of breathing. It is possible only after a reasonable mastery of asana practice.
Source: English translation of the Yoga Sutras of Patanjali (Swami Jnaneshvara Bharati)
Once that perfected posture has been achieved, the slowing or braking of the force behind, and of unregulated movement of inhalation and exhalation is called breath control and expansion of prana (pranayama), which leads to the absence of the awareness of both, and is the fourth of the eight rungs.