Musnad Abu Ya'la al-Mawsili 152:262

Abu Khaythama narrated to us, Ahmad ibn Ishaq narrated to us, Abd al-Aziz ibn Muslim narrated to us, Sulayman al-A'mash narrated to us, from Ibrahim al-Nakha'i, from Alqama, from Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "He shall not enter the Fire who has in his heart the weight of a mustard seed of faith, and he shall not enter Paradise who has in his heart the weight of a mustard seed of pride."