Manusmriti

siva.sh/manusmriti/9/126

जन्मज्येष्ठेन चाह्वानं सुब्रह्मण्यास्वपि स्मृतम् । यमयोश्चैव गर्भेषु जन्मतो ज्येष्ठता स्मृता ॥ १२६ ॥

sanskrit

And with respect to the Subrahmanya (texts) also it is recorded that the invocation (of Indra shall be made) by the first-born, of twins likewise, (conceived at one time) in the wombs (of their mothers) the seniority is declared (to depend) on (actual) birth.

english translation

janmajyeSThena cAhvAnaM subrahmaNyAsvapi smRtam | yamayozcaiva garbheSu janmato jyeSThatA smRtA || 126 ||

hk transliteration by Sanscript