Manusmriti

siva.sh/manusmriti/9/107

यस्मिनृणं संनयति येन चानन्त्यमश्नुते । स एव धर्मजः पुत्रः कामजानितरान् विदुः ॥ १०७ ॥

sanskrit

That son alone on whom he throws his debt and through whom he obtains immortality, is begotten for (the fulfilment of) the law; all the rest they consider the offspring of desire.

english translation

yasminRNaM saMnayati yena cAnantyamaznute | sa eva dharmajaH putraH kAmajAnitarAn viduH || 107 ||

hk transliteration by Sanscript