Devibhagavatapurana 11.19.9

Devībhāgavatapurāṇa 11.19.9

atastu kāraṇādvipraḥ sandhyāṃ kuryātprayatnataḥ |
sandhyayorubhayornityaṃ gāyatryā praṇavena ca ||