Manava Dharmashastra 9.117

9.117

ekādhikaṃ harej jyeṣṭhaḥ putro 'dhyardhaṃ tato 'nujaḥ
aṃśam aṃśaṃ yavīyāṃsa iti dharmo vyavasthitaḥ //

Let the eldest son take one share in excess, the (brother) born next after him one (share) and a half, the younger ones one share each; thus the law is settled.