It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever is given a gift, let him accept it and also repay it. If he does not have the means to repay it, let him praise him. Whoever praises him has, as it were, thanked him, and whoever conceals it has been ungrateful to him."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الهبات / 12030