Bhagavad Gita 6.13
Let him hold his body, head, and neck erect, motionless and steady; let him look fixedly at the tip of his nose, turning neither to the right nor to the left.
समं कायशिरोग्रीवं धारयन्नचलं स्थिरः।
संप्रेक्ष्य नासिकाग्रं स्वं दिशश्चानवलोकयन्॥samaṃ kāyaśirogrīvaṃ dhārayannacalaṃ sthiraḥ|
saṃprekṣya nāsikāgraṃ svaṃ diśaścānavalokayan||
English translation (above): Shri Purohit Swami (1935), public domain. Sanskrit text via github.com/gita/gita (Unlicense).