The Law Code of Yājñavalkya 2.8.46

2.8.46

yadā vibhāgaṃ pitā cikīrṣati tadā icchayā vibhajet putrān ātmanaḥ sakāśāt putraṃ putrau putrān | icchāyā niraṅkuśatvād aniyamaprāptau niyamārtham āha jyeṣṭaṃ vā śreṣṭhabhāgena iti | jyeṣṭaṃ śreṣṭhabhāgena madhyamaṃ madhyabhāgena kaniṣṭḥaṃ kaniṣṭhabhāgena vibhajed ity anuvartate | śreṣṭhādivibhāgaś ca manunoktaḥ:

When a father wishes to make a partition, then at his will he may divide the sons from himself—a son, two sons, [or] sons. Because desire is unchecked and irregularity would result, he states in order to establish a restriction: "either the eldest with the best share". The sentence continues: he should divide the eldest with the best share, the middle with the middle share, and the youngest with the least share. And the partition into the best and other shares has been stated by Manu: