From the chapters of Islamic Opening of Syria: The muslims are reminded of the times of Prophet in Jerusalem

[Source: The Sword of Allah - Khalid bin Al-Waleed By Late Lieutenant-General A.I. Akram of the Pakistan Army]

Scene: For four months the siege continued without a break. Then the Patriarch of Jerusalem, a man by the name of Sophronius, offered to surrender the city and pay the Jizya, but only on condition that the Caliph himself would come and sign the pact with him and receive the surrender. When the Patriarch's terms became known to the Muslims, Sharhabeel(R.A.) suggested that instead of waiting for Umar(R.A.) to come all the way from Madinah, Khalid(R.A.) should be sent forward as the Caliph. Umar(R.A.) and Khalid(R.A.) were very similar in appearance; and since the people of Jerusalem would only know Umar(R.A.) by reports, they could perhaps be taken in by a substitute. The Muslims would say that actually the Caliph was already there-and lo, he comes!
On the following morning the Patriarch was informed of the Caliph's presence, and Khalid(R.A.), dressed in simple clothes of the poorest material, as was Umar(R.A.)'s custom, rode up to the fort for talks with the Patriarch. But it did not work. Khalid(R.A.) was too well known, and there may have been Christian Arabs in Jerusalem who had visited Madinah and seen both Umar(R.A.) and Khalid(R.A.), noting the differences. Moreover, the Patriarch must have wondered how the great Caliph happened to be there just when he was needed! Anyhow, the trick was soon discovered, and the Patriarch refused to talk. When Khalid(R.A.) reported the failure of this mission, Abu Ubaidah(R.A.) wrote to Umar(R.A.) about the situation, and invited him to come to Jerusalem and accept the surrender of the city. In response the Caliph rode out with a handful of Companions on what was to be the first of his four journeys to Syria.


Umar(R.A.) sat with a large group of Companions, talking of this and that. Soon it would be time for the early afternoon prayer. Bilal(R.A.) the Negro was also present. Bilal(R.A.), had suffered many tortures in the early days of Islam at the hands of the unbelieving Qureish, but had remained steadfast in his faith. When the institution of the Adhan (the Muslim call to prayer) was adopted in 2 Hijri, the Prophet(S.A.W.) appointed Bilal(R.A.) as the Muazzin; and thereafter, five times a day, the powerful and melodious voice of Bilal(R.A.) could be heard at Madinah, calling the Faithful to prayer. Over the years Bilal(R.A.) had risen in stature as a saintly Muslim, and had become one of the closest and most venerated Companions of the Prophet(S.A.W.). But on the death of the Holy Prophet(S.A.W.), Bilal(R.A.) had fallen silent; he would not call the Adhan any more.
It now occurred to some of the Companions that perhaps the conquest of the holy city of Jerusalem was an important enough occasion for Bilal(R.A.) to break his silence. They asked Umar(R.A.) to urge him to call the Adhan, just this one time! Umar(R.A.) turned to Bilal(R.A.): "O Bilal! The Companions of the Messenger of Allah implore you to call the Adhan and remind them of the time of their Prophet, on whom be the blessings of Allah and peace." 1 For a few moments Bilal(R.A.) remained lost in thought. Then he looked at the eager faces of the Companions(R.A.) and at the thousands of Muslim soldiers who were gathering for the congregational prayer. Then he stood up. Bilal(R.A.) would call the Adhan again!
The glorious voice of the illustrious Muazzin beat upon the vast multitude. As he called the opening words, Allah is Great, the minds of the Faithful turned to memories of the dearly loved Prophet Muhammad (may peace and blessings of Allah be upon him) and tears welled up in their eyes. When Bilal came to the words, Muhammad is the Messenger of Allah, his audience broke down and sobbed.


1. Waqidi: p. 165.