口は災いのもと The Mouth is the Origin of Disasters

口は災いのもと (or 口は禍のもと) The mouth is the origin of disasters

口は禍のもとだから、噂話はやめたほうがいい。
kuchi wa wazawai no moto dakara uwasa banashi wa yameta hou ga ii.
The mouth is the origin of disasters, therefore you should stop gossiping.

口 kuchi - mouth
は wa - (topic particle)
禍 wazawai - calamity; disaster; catastrophe
だから dakara - so; therefore
噂話 uwasa banashi - gossip (噂 uwasa rumor; hearsay + 話 hanashi talk; story - the 'h' takes a harder 'b')
やめたほうがいい yameta hou ga ii - should (ought to) stop