Sayyiduna Ibn Abbas (may Allah be pleased with them both) said concerning this statement of Allah Almighty: "And do not confer favour seeking to gain more" [Al-Muddaththir: 6]: "That is, do not give anyone something in the hope that he will give you more than it."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12618