Shiva Purana
हुत्वोपरिष्टात्तन्त्रं च तेनाग्नेरुत्तरे बुधः ॥ स्थित्वासने जपेन्मौनी चैलाजिनकुशोत्तरे ॥ यावद्ब्राह्ममुहूर्त्तं तु गायत्रीं दृढमानसः ॥ ५२॥
Having offered oblations to the Lord with the tantra on the north side of the fire Standing on a seat, one should chant the mantra in silence on a cloth, deerskin and kusha grass. One should chant the Gāyatrī with a firm mind until the Brahma Muhūrta.
english translation
hutvopariSTAttantraM ca tenAgneruttare budhaH ॥ sthitvAsane japenmaunI cailAjinakuzottare ॥ yAvadbrAhmamuhUrttaM tu gAyatrIM dRDhamAnasaH ॥ 52॥
hk transliteration by Sanscript