Bhagavad Gita 6.11

शुचौ देशे प्रतिष्ठाप्य स्थिरमासनमात्मन: |
नात्युच्छृतं नातिनीचं चैलाजिनकुशोत्तम

Translation

To practice Yog, one should make an āsan (seat) in a sanctified place, by placing Kuśh grass, deer skin, and a cloth, one over the other. The āsan should be neither too high nor too low.