يُوَسْوِسُ فِي صُدُورِ النَّاسِ
He whispers into the breasts of humankind. (114:5)
Fī is a preposition; ṣudūr is its genitive complement and is annexed to al-nās. The phrase qualifies the whispering and portrays it as affecting the inner person rather than merely sounding externally.
The rhetorical image concerns inward thought and inclination. It does not, by itself, establish an unstated physiological mechanism.