Answer
Sa'd ibn Abi Waqqas was dismissed from his post by the new caliph, Umar ibn al-Khattab. He had been appointed governor of Iraq by the previous caliph, Abu Bakr, but Umar felt that he was not being sufficiently strict in enforcing the laws of Islam. Sa'd was also accused of being too close to the Persians, who had recently been conquered by the Muslims.