Tafsir of Al-Insan 76:9

{We feed you only for the sake of Allah}—this is said upon the intention that the statement is in the position of a circumstantial qualifier (hal) for the subject of "they feed." That is, they say this with the tongue of their state, due to the signs of sincerity that appear upon them. It is reported from Mujahid that they did not actually speak these words, but Allah Almighty knew it from them and He, glory be to Him, stated it on their behalf so that those who desire [reward] may aspire to it. Or, [they said it] with the tongue of speech to remove any suspicion of bestowing favors, which invalidates charity, and to negate the expectation of compensation, which diminishes the reward.

It is reported from the Siddiqah (may Allah be pleased with her) that she used to send charity to a household and then ask the messenger, "What did they say?" If he mentioned that they prayed for her, she would pray for them with the like of it, so that the reward of the charity would remain pure for her with Allah, the Almighty and Majestic. It has been permitted that this statement of theirs to them is a form of kindness, instruction, and alerting them to how one who is sincere to Allah Almighty should be—though that is not the preferred view.

His statement, glory be to Him: {We desire from you neither reward}—in terms of deeds—{nor gratitude}—nor thanks and praise—in terms of words, is a confirmation and emphasis of what preceded it.