Zirr bin Hubaysh (may Allah have mercy on him) narrated: Sayyiduna Ibn Abbas (may Allah be pleased with them both) asked me: Which of the two readings do you recite? I said: The last one. He said: "Jibril (peace be upon him) used to review the Qur'an with the Prophet (peace and blessings be upon him) every year in Ramadan, and in the year in which the Prophet (peace and blessings be upon him) passed away, he reviewed the Qur'an with him twice." So Sayyiduna Abdullah (bin Mas'ud (may Allah be pleased with him)) witnessed all that was abrogated and all that was changed, and so the reading of Abdullah is the last reading.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 12549