Sincerity in calling to Allah (da'wah)

أَمْ تَسْـَٔلُهُمْ خَرْجًا فَخَرَاجُ رَبِّكَ خَيْرٌ ۖ وَهُوَ خَيْرُ ٱلرَّٰزِقِينَ

Or do you, [O Muḥammad], ask them for payment? But the reward of your Lord is best, and He is the best of providers.