Speak good, or be silent
مَنْ كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ فَلْيَقُلْ خَيْرًا أَوْ لِيَصْمُتْ
Man kana yu'minu billahi wa-l-yawmi-l-akhir fa-l-yaqul khayran aw li-yasmut.
"Whoever believes in Allah and the Last Day, let him speak good or remain silent."
Narrated by Abu Hurayrah. Sahih al-Bukhari 6018, Sahih Muslim 47 · Nawawi's Forty · #15
A short reflection
The Prophet ﷺ made watching the tongue a test of faith. The highest bar on the simplest behaviour.