"Whoever is given a gift, then if he finds something let him reciprocate. If he has nothing, then let him mention some praise. For whoever has mentioned some praise, then he has expressed his gratitude. And whoever refrains (from doing so) then he has committed Kufr. And whoever pretends to be satisfied by that which he was not given, he is like the one who wears a garment of falsehood."
Reference: Jami` at-Tirmidhi 2034 In-book : Book 27, Hadith 140 English translation : Vol. 4, Book 1, Hadith 2034
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