Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: The Prophet (peace and blessings be upon him) established brotherhood between Sayyiduna al-Zubayr (may Allah be pleased with him) and Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him).
Al-Tabarani recorded it in al-Mu'jam al-Awsat and al-Mu'jam al-Kabir; the narrators of al-Mu'jam al-Awsat are thiqat (trustworthy).