Yoga Rahasya Chapter One verse 34

reca-pūraka-kumbheṣu mano’nusaraṇaṃ smṛtam |
reca-pūraka-kumbhākhyāḥ sarve prāṇavidhārakāḥ ||

Remember the mind should follow the exhale, inhale and retention.
Exhale, inhale and retention all support the vital force.

recaka - exhalepūraka - inhalekumbhaka - retentionmanas - mind; that which objects of sense affect; the faculty or instrument through which thoughts enteranu - follow, one after another, followingsmṛti - remembrance; memory; mindfulness; the whole body of sacred tradition or what is remembered by human teachers and constantly revisedsarva - whole, entire, all, every one; altogether, wholly, completely, in all parts, everywhereprāṇa - the breath of life, breath, respiration, spirit, vitality; vital air; air inhaled, wind; vigour, energy, power

Commentaries and Reflections

Commentary by TKV Desikachar:

Mind should follow the breath.
ExhaleInhale and Retention support life.
So during Āsana it is desirable that the mind must follow them.”