Brahma Sutras
ॐ नाणुरतत् श्रुतेरिति चेन्न इतराधिकारात् ॐ ॥ २-३-२२॥
And the individual soul is atomic because of the direct Upanishadic use of the word as well as mention of infinitesimality.
english translation
OM nANuratat zruteriti cenna itarAdhikArAt OM ॥ 2-3-22॥
hk transliteration by Sanscript