Hathayoga 1-11

हठ-विद्या परं गोप्या योगिना सिद्धिमिच्छता ।

भवेद्वीर्यवती गुप्ता निर्वीर्या तु प्रकाशिता ॥ ११॥

Hathavidyā param gopyā yoginā siddhimichchatā bhavedvīryavatī guptā nirvīryā tu prakāśitā

A Yogî desirous of success should keep the knowledge of Hatha Yoga secret; for it becomes potent by concealing, and impotent by exposing.