Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 5
मौनं सदासनजय: स्थैर्यं प्राणजय: शनै: ।प्रत्याहारश्चेन्द्रियाणां विषयान्मनसा हृदि ॥ ५ ॥
maunaṁ sad-āsana-jayaḥsthairyaṁ prāṇa-jayaḥ śanaiḥpratyāhāraś cendriyāṇāṁviṣayān manasā hṛdi
One must observe silence, acquire steadiness by practicing different yogic postures, control the breathing of the vital air, withdraw the senses from sense objects and thus concentrate the mind on the heart.
The yogic practices in general and haṭha-yoga in particular are not ends in themselves; they are means to the end of attaining steadiness. First one must be able to sit properly, and then the mind and attention will become steady enough for practicing yoga. Gradually, one must control the circulation of vital air, and with such control he will be able to withdraw the senses from sense objects. In the previous verse it is stated that one must observe celibacy. The most important aspect of sense control is controlling sex life. That is called brahmacarya. By practicing the different sitting postures and controlling the vital air, one can control and restrain the senses from unrestricted sense enjoyment.