مَنْ كَانَ يُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الْآخِرِ فَلْيَقُلْ خَيْرًا أَوْ لِيَصْمُت
Whoever truly believes in Allaah and in the Last Day, then he should
speak good or else remain silent.
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(Taken from Hadeeth 15 of Nawawi’s 40. Al-Bukhari and Muslim)