Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates this saying of the Prophet (peace and blessings be upon him): "Providing food on one day at a wedding is Sunnah, providing it for two days is extra, and providing it for three days is showing off and seeking reputation."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains a narrator, Muhammad bin Ubaydullah al-Arzami, and he is da'if (weak).