How can Omar's refusal to carry out the Prophet's command be justified, despite Allah's saying: "It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair"?

Umar ibn al-Khattab is one of the Rightly Guided Caliphs, whose Sunnah the Prophet, peace be upon him, commanded us to follow. The Companions differed in their understanding of some of the Prophet’s commands, peace be upon him, such as the incident of the Asr prayer on the Day of Al-Ahzab, where the Prophet did not reprimand either of the two groups for their ijtihad (independent reasoning). Umar and the Companions with him refrained from giving the Prophet, peace be upon him, what he requested, understanding that the command was a guidance and not an obligation, and out of compassion for the Prophet. The mujtahid (one who performs ijtihad) is rewarded even if he errs in his ijtihad.