Sraddhatraya Vibhaga Yoga-Bhagavad Gita Chapter 17 [The Three Divisions of Material Existence]
Krishna Said > Shaloka: 14
The austerity of the body consists in this: worship of the Supreme Lord, the brahmanas, the spiritual master, and superiors like the father and mother. Cleanliness, simplicity, celibacy and nonviolence are also austerities of the body.
The Supreme Godhead here explains the different kinds of austerity and penance. First He explains the austerities and penances practiced by the body. One should offer, or learn to offer, respect to God or to the demigods, the perfect, qualified brahmanas and the spiritual master and superiors like father, mother or any person who is conversant with Vedic knowledge. These should be given proper respect. One should practice cleansing oneself externally and internally, and he should learn to become simple in behavior. He should not do anything which is not sanctioned by the scriptural injunctions. He should not indulge in sex outside of married life, for sex is sanctioned in the scripture only in marriage, not otherwise. This is called celibacy. These are penances and austerities as far as the body is concerned.