Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates that the Prophet (peace and blessings be upon him) said: "On the evening of Arafah, every person who has even a mustard seed's weight of faith in his heart is forgiven." The narrator says: I said: "O Messenger of Allah, is this specific to the people of Arafah?" The Prophet (peace and blessings be upon him) said: "(No,) rather, it is general for the Muslims."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Abu Dawud al-A'ma (the blind), who is extremely da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الحج / 5551