Manava Dharmashastra 4.94

4.94

yadartho'yaṃ punarvidhistaddarśayati | āyurādiphalakāmo dīrghakālasandhyājapaṃ kuryāt | satyapi nityatve dairghyādguṇātphalamidam | anagnikasya proṣitasyaitatsambhavati | anyasya tvagnihotrakāloparodho dīrghasandhyāvidhisampādanāt | ‘dīrghasandhyā’ guṇata ucyate | sandhyāsahacarite japāvidhau sandhyāśabdo vartate | dīrghā sandhyaiṣāmiti bahuvrīhiḥ | ṛṣigrahṇamarthavādaḥ |

It was by reason or their prolonged twilight-devotions that the sages obtained long life, wisdom, fame, refutation and Brahmic glory.—(94)

Medhātithi’s commentary (manubhāṣya):
The Author proceeds to show for what purpose the aforesaid act has been prescribed. The meaning is that if one desires such rewards as long life and so forth, he should perform the twilight-devotions for a long time. Though the act is an obligatory one, yet the said rewards follow from prolonging it.

This prolongation is possible only for one who is either without Fires, or is away from home. As for others (e.g., the man with the Fires living at home), the prolongation of the Twilight-Devotions would inpinge upon the time laid down for the Agnihotra-offerings.

The twilight devotion is called ‘prolonged,’ only figuratively; the term ‘twilight devotion’ standing for the reciting of Mantras, etc., that is done in connection with the said devotion.

The compound ‘dīrghasandhyā’ is a Bahuvrīhi.

The mention of the sages is by way of a commendatory statement—(94)

This verse is quoted in Nityācārapradīpa (p. 386), which explains that ‘dīrghasandhyatva’ is secured by continuing the Japa till after sunrise.

Comparative notes by various authors
Mahābhārata (Anuśāsana, 104.18).—[Same as Manu, reading ‘nityasandhya’ for ‘dīrghasandhya.’]

Yama (in Parāśaramādhava, p. 300).—[Same as Manu.]