Patanjali Yoga Sutra 2.47

Sanskrit Sutra

प्रयत्नशैथिल्यानन्तसमापत्तिभ्याम्॥४७॥

Swami Vivekananda's translation

By lessening the natural tendency (for restlessness) and meditating on the unlimited, posture becomes firm and pleasant.

Chapter link: Yoga Sutra Chapter 2