Bhagavad Gita 6.11 Chapter 6, Verse 11

शुचौ देशे प्रतिष्ठाप्य स्थिरमासनमात्मनः ।
नात्युच्छ्रितं नातिनीचं चैलाजिनकुशोत्तरम् ॥६.११॥

śucau deśe pratiṣṭhāpya sthiramāsanamātmanaḥ
nātyucchritaṃ nātinīcaṃ cailājinakuśottaram

Meaning

In a clean place, let him set up a firm seat, neither too high nor too low, covered with kusha grass, a deerskin and a cloth.

Newly written by Bhajan Dhun — scholarly review in progress.

Reference: Bhagavad Gita 6.11 (Chapter 6, The Yoga of Meditation (Atma Samyama Yoga), Verse 11)