Shiva Purana
गायत्रीं शिरसा सार्द्धं जपेद्व्याहृतिपूर्विकाम् ॥ त्रिवारमायतप्राणाः प्राणायामस्स उच्यते ॥ ९॥
Holding the breath one shall repeat the Gāyatrī Mantra along with his Śiras (Oṃ) and Vyāhṛtis thrice. That is called Prāṇāyāma.
english translation
gAyatrIM zirasA sArddhaM japedvyAhRtipUrvikAm ॥ trivAramAyataprANAH prANAyAmassa ucyate ॥ 9॥
hk transliteration by Sanscript