That old man Ajāmila had ten sons, of whom the youngest was a baby named Nārāyaṇa. Since Nārāyaṇa was the youngest of all the sons, he was naturally very dear to both his father and his mother.
Srimad Bhagavata Purana
श्रीमद्भागवतमहापुराणम्
मूल श्लोकः
तस्य प्रवयसः पुत्रा दश तेषां तु योऽवमः । बालो नारायणो नाम्ना पित्रोश्च दयितो भृशम्
tasya pravayasaḥ putrā daśa teṣāṃ tu yo'vamaḥ | bālo nārāyaṇo nāmnā pitrośca dayito bhṛśam
SB 6.1.24