Quran on Dawah: Surah Aal-e-Imran: 104

"Let there arise out of you a group of people inviting to all that is good, enjoining what is right and forbidding what is wrong, and it is they who are succesful."

Video: Shaykh Yusuf Estes: How to Invite Non-Muslims to Islam

I Defeat my Opponents with Three

It was related that a man said to Haatim al Asam: “You are a non-Arab who cannot speak fluently, but no one has debated you except that you silenced him, so how do you defeat your opponents?”

He said: “With three:

1. I am pleased when my opponent is correct,
2. I am sad when he is incorrect,
3. And I preserve my tongue from saying anything that offends him.”

Al Imaam Ahmad stated: “What a wise man he is.”

Al Khateeb al Baghdaadee stated: “A person’s intention should be to clarify the truth when debating, not to defeat his opponent.”

Al Imaam ash Shaafi’ee said: “I did not debate anyone while hoping that he would err.”

He also said: “I never spoke to anyone except that I hoped that he would be granted success and supported. I never spoke to anyone except that I didn’t mind whether Allaah clarified the truth upon my tongue or his.”

Al Hafidh ibn Rajab comments: “This indicates that he did not have any intention except the clarification of the truth, even if it were from the person who is debating and contradicting him. Whoever is of this state would not dislike it when his opinions are refuted and when his contradiction to the Sunnah is clarified, not while alive or after his death. This is also what we think of the other Imaams of Islaam, those who defended Islaam and supported it whether they were from the Salaf or those who came after them. They also never used to dislike those who contradicted them as long as they were following evidence, even though the evidence was not strong enough for them to follow or leave their own evidence for.”

References:
The Difference between Advising and Degrading by Ibn Rajab al Hanbalee.
Al Faqee wal Mutafaqih by al Khateeb al Baghdaadee.

Quran on Dawah: An-Nahl: 125

“Invite to the Way of your Lord with wisdom and fair preaching, and argue with them in a way that is better. Truly, your Lord knows best who has gone astray from His Path, and He is the Best Aware of those who are guided.” (Quran, an-Nahl: 125)