قُلْ مَا سَأَلْتُكُم مِّنْ أَجْرٍ فَهُوَ لَكُمْ ۖ إِنْ أَجْرِىَ إِلَّا عَلَى ٱللَّهِ ۖ وَهُوَ عَلَىٰ كُلِّ شَىْءٍ شَهِيدٌ Qur’an Saba (34:47)Qul m a saaltukum min ajrin fahuwa lakum in ajriya ill a AAal a All a hi wahuwa AAal a kulli shayin shaheed un
I.e., no reward of a material nature: cf. 25:57 - "no reward other than that he who so wills may unto his Sustainer find a way".
Cf. x. 72. The second argument is that he has nothing to gain from them. His message is for their own good. He is willing to suffer persecution and insult, because he has to fulfil his mission from Allah.