Bhagavad Gita 6.11
Having chosen a holy place, let him sit in a firm posture on a seat that is neither too high nor too low, and cover it with a grass mat, a deer skin, and a cloth.
शुचौ देशे प्रतिष्ठाप्य स्थिरमासनमात्मनः।
नात्युच्छ्रितं नातिनीचं चैलाजिनकुशोत्तरम्॥śucau deśe pratiṣṭhāpya sthiramāsanamātmanaḥ|
nātyucchritaṃ nātinīcaṃ cailājinakuśottaram||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).