A kindly tongue is

Consort with all men, O people of Bahá, in a spirit of friendliness and fellowship. If ye be aware of a certain truth, if ye possess a jewel, of which others are deprived, share it with them in a language of utmost kindliness and good-will. If it be accepted, if it fulfil its purpose, your object is attained. If any one should refuse it, leave him unto himself, and beseech God to guide him. Beware lest ye deal unkindly with him.
A kindly tongue is the lodestone of the hearts of men. It is the bread of the spirit, it clotheth the words with meaning, it is the fountain of the light of wisdom and understanding….

Gleanings From the Writings of Bahá’u’lláh