Manava Dharmashastra 4.93

4.93

utthāyāvaśyakaṃ kṛtvā kṛtaśaucaḥ samāhitaḥ
pūrvāṃ saṃdhyāṃ japaṃs tiṣṭhet svakāle cāparāṃ ciram //

When he has risen, has relieved the necessities of nature and carefully purified himself, let him stand during the morning twilight, muttering for a long time (the Gayatri), and at the proper time (he must similarly perform) the evening (devotion).