It is an excellent rule to be observed in all disputes, that men should give soft words and hard arguments; that they should not so much strive to vex as to convince each other. John Wilkins
Soft words soften the hearts that are harder than rock, harsh words harden hearts that are softer than silk. Abu Hamid al-Ghazali
Use soft words in hard arguments. Henry George Bohn
Soft words butter no parsnips, but they won't harden the heart of the cabbage either. Irish Proverb
Soft words butter no turnips. Irish Proverb
Soft words and hard arguments. Portuguese Proverb