Mudgalapurana 3.18.20

3.18.20

vānaprasthaśca kāle vai sahasraṃ japamācaret |
evaṃ sandhyā trikāleṣu brāhmaṇaiḥ kriyate sadā ||

And a forest-dweller should perform a thousand recitations at that time. / In this way, the twilight worship is always performed at the three times by brahmins.