By Patanjali 
The Yoga Sutras of Patanjali [EPUB ebook] 

The Yoga Sūtras of Patañjali are Indian sūtras (aphorisms) that constitute the foundational text of yoga. In medieval times, yoga was cast as one of the six orthodox āstika schools of Hindu philosophy.
The Yoga Sutras form the theoretical and philosophical basis of Rāja Yoga, and are considered to be the most organized and complete definition of that discipline. The Sutras not only provide yoga with a thorough and consistent philosophical basis, they also clarify many important esoteric concepts which are common to all traditions of Indian thought, such as karma.