Vijnana Bhairava Tantra
वायुद्वयस्य सम्घट्टाद् अन्तर् वा बहिरन्ततः । योगी समत्वविज्ञानसमुद्गमनभाजनम् ॥ ६४ ॥
From the fusion of both vayus (prana and apana) inside or outside (the body), the yogi attains equilibrium and becomes fit for the proper manifestation of consciousness.
english translation