Defile not your tongues with the cursing and reviling of any soul

O people of Bahá! Ye are the dawning-places of the love of God and the daysprings of His loving-kindness. Defile not your tongues with the cursing and reviling of any soul, and guard your eyes against that which is not seemly.

- Baha'u'llah

http://reference.bahai.org/en/t/b/TB/tb-4.html