sayam pratar upasita
sandhye ubhe ca yata-vag
japan brahma samahitah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
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 Visnu and by chanting the Gayatri mantra he should worship them.