Manava Dharmashastra 2.101

2.101

pūrvāṃ saṃdhyāṃ japāṃs tiṣṭhet sāvitrīm ārkadarśanāt
paścimāṃ tu samyag ṛkṣavibhāvanāt //

Let him stand during the morning twilight, muttering the Savitri until the sun appears, but (let him recite it), seated, in the evening until the constellations can be seen distinctly.