Thirukkural – 882
Fear not sword like overt foes,
fear friendship of kinsmen-like foes.
வாள்போல் பகைவரை அஞ்சற்க வஞ்சுக
கேள்போல் பகைவர் தொடர்பு
One can protect oneself from visible sword like known enemies, so he need not fear them much. But one has to fear enemies who pretend to be kin yet harbor enmity in their hearts. He has to be more careful with them than open foes.
The original verse does not explicitly state ‘overt’. I have added it in translation for better readability.
வாள் – Sword
கேள் – Relatives