Musnad Ahmad 28:50

Ya'la ibn 'Ubayd narrated to us, Abu Hayyan narrated to us, from his father, who said: 'Abdullah ibn 'Amr and 'Abdullah ibn 'Umar met. Then 'Abdullah ibn 'Umar approached while he was weeping. The people said to him: "What makes you weep, O Abu 'Abd al-Rahman?" He said: "That which this [man] narrated to me. He said: I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'No person shall enter Paradise who has in his heart the weight of a mustard seed of pride.'"