Srimad-Bhagavatam: Canto 7 - Chapter 1 - Verse 2
Sanskrit:
सायं प्रातरुपासीत गुर्वग्न्यर्कसुरोत्तमान् ।सन्ध्ये उभे च यतवाग्जपन्ब्रह्म समाहित: ॥ २ ॥
ITRANS:
sāyaṁ prātar upāsītagurv-agny-arka-surottamānsandhye ubhe ca yata-vāgjapan brahma samāhitaḥ
Translation:
At both junctions of day and night, namely, in the early morning and in the evening, he should be fully absorbed in thoughts of the spiritual master, fire, the sun-god and Lord Viṣṇu and by chanting the Gāyatrī mantra he should worship them.