Commentary
وَمَا أَسْأَلُكُمْ عَلَيْهِ مِنْ أَجْرٍ
And I do not claim from you any reward for it. - 26:109
It is learnt from this verse that charging and acceptance of wages against religious teachings and preaching is not right. Therefore, the righteous elders have ruled it as forbidden, but the later generations have allowed it under compelling circumstances. Its full details have been given under the explanation of the verse لَا تَشْتَرُوا بِآيَاتِي ثَمَنًا قَلِيلًا do not take a paltry price for My signs" - 2:41.
Special note
Here the verse فَاتَّقُوا اللَّـهَ وَأَطِيعُونِ (So, fear Allah and obey me - 110) is repeated twice to make clear that for the obedience of the Messenger and fear of Allah it was enough that the prophet has any one of the two qualities: One, his honesty and integrity, and the second, his teaching and preaching without a fee. But when the Messenger who is personified of all these traits, his obedience and the fear of his God becomes all the more necessary.