Shaikh Saalih al-Fawzaan 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