I came across this verse in Utpala’s commentary to BS 64/65.62. I should appreciate if anyone could provide me with its source:

savyāhṛtiṃ sapraṇavāṃ gāyatrīṃ śirasā saha/

triḥ pāṭhed āyataprāṇaḥ prāṇāyāmaḥ sa ucyate//

Many thanks,

Ken