The Law Code of Yājñavalkya 2.8.71

2.8.71

uddhāre 'nuddhṛte tv eṣām iyaṃ syād aṃśakalpanā |
ekādhikaṃ harej jyeṣṭhaḥ putro 'dhyardhaṃ tato 'nujaḥ ||
aṃśam aṃśaṃ yavīyāṃsa iti dharmo vyavasthitaḥ ||
iti | (MDh 9.116–17)

"If a special share is not taken out, however, this would be the determination of their portions: / the eldest son shall take one extra [share], the next born one and a half, / and the younger sons one share each; such is the settled law." (MDh 9.116–17)