The Night Journey - 17:28
If you have to turn away from them in order to obtain a mercy from your Lord which you hope for, speak a soft word to them.
It was the attitude of all the messengers to be kind and gentle:
The Family Of Imran/3:159
You were kind to them as a gift from Allah. Had you been rude and hard-hearted, they would have scattered around you. Forgive them, ask for their forgiveness and consult them about what to do. And when you have decided, trust in Allah; Allah loves those who trust.
The Women/4:148
Allah does not like bad language. Except for those who are wronged. Allah is All-Hearing, All-Knowing.
The Bee/16:125
Call to the Way of your Lord with wisdom and good enlightenment. Argue with them in the best way. Your Lord knows best those who deviate from His way and those who are on the right path.
Ta-Ha/20:43
Go to Pharaoh, the two of you, because he was wicked.
Ta-Ha/20:44
Use a soft tongue with him, that he may take counsel or respect.
The Spider/29:46
"Argue with the people of the Book in the best way, except the unjust, and say: "We believe in what has been sent down to us and what has been sent down to you. Our God and your God is one. To Him alone have we submitted."