Translation:

40. Allah is never unjust in the least degree: if there is any good (done) He doubleth it and giveth from His own presence a great reward.

Notes (Tafseer)

559. Any little good of our own comes from the purity of our heart. Its results in the world are doubled and multiplied by Allah's grace and mercy; but an even greater reward comes from Him: His good pleasure, which brings us nearer to Him.