It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever believes in Allah and the Last Day, let him not harm his neighbour; and whoever believes in Allah and the Last Day, let him speak good or remain silent."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10797