Usamah b. Zayd was appointed the commander of an army by the Prophet and was ordered to proceed towards Syria to crush the aggression of the enemy. The Prophet had declared that anyone who did not pay heed to the command of Usamah was a culprit. Thus, at the time of the death of the Prophet and the proceedings of Saqifah, Usamah was away from Madinah at a place called Jurf (near Madinah), proceeding towards Syria (following the command of the Prophet).
The contents of the letter addressed to him were as follows, “From Abu Bakr, the caliph of the Prophet of Allah to Usamah b. Zayd. Now then! The Muslims have taken refuge in me and have chosen me for the leadership of caliphate and made me their chief after the Prophet of Allah.” (The letter being lengthy, until he reached here), “Thus when my letter reaches you, you may come to me, similar to the other Muslims and swear allegiance to me. Then you may even permit ‘Umar b. al-Khattab to be free from under your leadership and stay with me here for I need him. Then you may proceed towards the frontline as directed by the Prophet.”
When the letter reached Usamah, he read it and replied, “From Usamah b. Zayd, the freed slave of the Prophet of Allah, to Abu Bakr b. Abu Qahafah. Now then! Your letter has reached me, but it is completely incompatible from the beginning until the end. First you claim to be the caliph of the Prophet and then say that the Muslims gathered around you and chose you as their leader? If it was such, then they would have pledged allegiance at your hands in the Masjid and not at the Saqifah. Furthermore you request me to excuse ‘Umar b. al-Khattab from the army for the reason that you need him! Know that he has already stayed away on his own without my permission and it is not lawful for me to excuse anyone for the Prophet himself had deputed them for this battle under my command. In this matter there is no difference between you and ‘Umar, that both of you have stayed behind, and violation of the Prophet’s command after his death is similar to disobeying him during his lifetime. You very well know that the Prophet had ordered you and ‘Umar to proceed towards this expedition under my command while the opinion and command of the Prophet regarding you is better and preferable to your own opinions regarding yourselves. Your position was not hidden from the Prophet who made me your commander and not you my commander. Thus opposing the Prophet is hypocricy and deceit…”
House of Sorrows Translation of Baytul AhzanShaikh Abbas Al – Qummi