(And keep your opinion secret or proclaim it…) [67:13].
Said Ibn ‘Abbas: “This was revealed about the idolaters who used to slander the Messenger of Allah, Allah bless him and give him peace. Gabriel, peace be upon him, informed him about what they said about him and the things with which they slandered him. Some of them used to say to others: ‘Lower your voices lest Muhammad’s Allah hears you’ ”.
Asbab Al-Nuzul by Al-Wahidi , trans. Mokrane Guezzou