Shaikh Salih al-Fawzan hafidhuhallah said:

“…As for smiling, then it does not invalidate the prayer because it is not speech…”

[al-Muntaqa min Fatawa Fadilha Shaikh Salih al-Fawzan no. 106]

Translated by

Faisal Ibn Abdul Qaadir Ibn Hassan
Abu Sulaymaan