Musnad Ahmad 26:397

Aswad ibn Amir narrated to us, Abu Bakr informed us, from al-A’mash, from Ibrahim, from Alqama, from Abdullah, who said: The Messenger of Allah, peace and blessings of Allah be upon him, said: "No man shall enter Paradise who has in his heart the weight of a mustard seed of pride, and no man shall enter the Fire who has in his heart the weight of a mustard seed of faith."