It is narrated from Sayyida Aisha (may Allah be pleased with her) that the Messenger of Allah (peace and blessings be upon him) said: Whoever is done a favor, let him repay it; if he is unable to repay it, then let him mention it. Thus, whoever mentions it has expressed gratitude for it, and whoever displays a blessing which he does not possess is like a person who wears two garments of falsehood.
Hadith Referenceالفتح الربانی / مسائل الأخلاق الحسنة وما يتعلق بها / 9245
Hadith Gradingمحدثین:صحیح
Hadith Takhrij«صحيح، أخرجه الطبراني في الاوسط : 2484، وابن راھويه: 774 ، (انظر مسند أحمد ترقيم الرسالة: 24593 ترقیم بيت الأفكار الدولية: 25100»
Brief Explanation
Benefits: … What is meant by mentioning is that one should say words of praise regarding him, such as: “You have been very kind,” “You have shown compassion,” “Thank you very much,” and make supplication for him. As it is narrated from Sayyiduna Usamah bin Zayd radi Allahu anhu that the Messenger of Allah sallallahu alayhi wa sallam said: “Whoever good is done to, and he says to the doer, ‘Jazak Allahu khayran (May Allah reward you with goodness),’ he has indeed conveyed the utmost in praise.” … That is, whoever is treated with kindness and says to the one who did the kindness, “May Allah grant you a good reward,” he has fully expressed gratitude. (Tirmidhi: 1958) Thus, it is clear that the best words to use for supplication are “Jazak Allahu khayran.”
Wearing two garments of falsehood means that it is as if falsehood has covered the entire body of that person; he is expressing something about himself which is not actually his.