Umar ibn Abd al-Aziz , commonly known as Umar II , was the eighth Umayyad caliph. He is credited to have instituted significant reforms to the Umayyad central government, by making it much more efficient and egalitarian.
His rulership is marked by the first official collection of Hadith and the mandated universal education to the populace. He dispatched emissaries to China and Tibet, inviting their rulers to accept Islam. It was during his three-year reign that Islam was accepted by huge segments of the populations of Persia and Egypt. He also ordered the withdrawal of the Muslim forces in various fronts such as in Constantinople, Central Asia and Septimania. However despite this, his reign witnessed the Umayyads gaining many new territories in the Iberian Peninsula.
Due to this, Umar has been described as and was called the first Mujaddid and sixth righteous caliph of Islam after Hasan ibn Ali according to some Muslim scholars. Wikipedia
History of the Prophets and Kings, Vol. 24, p. 98/99, also quoted in Umar Bin Abd Al-Aziz, p. 708-710
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Letter to Abd al-Rahman bin Nu'aym, also quoted in History of the Prophets and Kings, Vol. 24, p. 101
Quran: Chapter 28, verse 83, quoted in History of the Prophets and Kings, Vol. 24, p. 102
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