Tantrasara (Krishnananda) 2.p92

2.p92

hrīṃ haṃsaḥ iti sūryārghyaṃ dattvā, tāmrādipātre
candanārkakusumāparājitā - puṣpāṇi nikṣipya,
udyadādityamaṇḍalavartinyai nityacaitanyoditāyai śrīmadekajaṭāyai
svāhā ityarghyaṃ dattvā, gāyatrīṃ japediti viśeṣaḥ .

Having offered the water offering to the Sun with “hrīṃ haṃsaḥ”, and having placed sandalwood, arkā flowers, and aparājitā flowers into a copper or other vessel, / having offered the water offering with “To the glorious Ekajaṭā, residing in the rising solar orb, arisen from eternal consciousness, svāhā”, he should recite the gāyatrī: this is the special rule.