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Abu Ubayda ibn al-Jarrah was a commander of the Rashidun Army, the Muslim army that conquered the Arabian Peninsula and expanded the Islamic Empire. He was one of the most trusted generals of Muhammad, and he led the Muslim forces in many battles, including the Battle of Yarmouk and the Battle of Khaybar. He was also a close friend of Umar ibn al-Khattab, the second caliph of Islam.