Yoga Sutra - Sādhana Pada 2.29
यम नियमासन प्राणायाम प्रत्याहार धारणा ध्यान समाधयोऽष्टावङ्गानि ॥२९॥
yama-niyama-āsana-prāṇāyāma-pratyāhāra-dhāraṇā-dhyāna-samādhayaḥ aṣṭau-aṅgāni ||29||
Restraint, observance, postures, control of breath, withdrawal, concentration, meditation, and samadhi are the eight limbs.
The eight limbs are abstentions, observances, posture, breath control, disengagement of the senses, concentration, meditation, and absorption.
In I.12, practice and dispassion were presented as the means to control the vṛttis and thus attain samādhi; in II.1, self-discipline, study, and submission to the Lord were identified as practices conducive to eliminating the kleśa obstacles to yoga and attaining samādhi. The following sūtras offer further prescriptions for attaining the goal of yoga. Actually, both sets of injunctions in I.12 and in II.1 can be located within the first two limbs, the yamas, abstentions, and niyamas, observances; practice and dispassion find correlates in the yamas; and the three ingredients of kriyā-yoga are repeated verbatim under the niyamas. Śaṅkara also adds that there are other requirements of the path, such as practicing dharma, righteous conduct, and accepting a guru. The commentators save their analyses of the eight items listed in this sūtra for the following sūtras, which take up each limb individually.
One might add, here, that the notion of yoga having aṇgas, limbs, is derived in all likelihood from the older Veda-aṇgas. The successful performance of Vedic ritual in the later Vedic period was seen as dependent on the mastery of the six limbs of the Vedic ritual: phonetics, meter, grammar, etymology, astronomy, and ritual (mentioned in, e.g., Muṇḍaka Upaniṣad I.1.5). Likewise, the goal of yoga expressed in I.2 is dependent on the successful performance of the eight auxiliary limbs of yoga indicated in this verse. Just as none of the Vedic limbs individually represented the goal of Vedic sacrifice, but each was an essential contributing part of it, so is the case with the limbs of yoga.