Performing Prayers in congregation is a confirmed Sunnah. Many hadiths discuss the virtue and importance of performing Prayers in congregation. Ibn `Umar quoted the Prophet (peace and blessings be upon him) as saying: “The reward of Prayer performed congregation outweighs that of Prayer performed individually by twenty-seven degrees” (Al-Bukhari and Muslim). It is permitted for the imam to prolong the first rak`ah while waiting for others to join the congregation.
Elaborating more on this, we cite what Sheikh Sayyed Sabiq states in his well-known book, Fiqh Us-Sunnah: It is permitted for the imam to prolong the first rak`ah while waiting for others to join the congregation. In the same way, it is preferred for him to wait for people who are coming during the bowings and during the final sitting.

Abu Qatadah reports that the Prophet (peace and blessings be upon him) used to prolong the first rak`ah and the people suspected that he did so to allow the latecomers to join the first rak`ah. Abu Sa`id says: “If the (congregational) Prayer was due to begin, you could go to al-Baqi`, relieve yourself, make wudu’, and return and find the Prophet still in the first rak`ah for he would prolong it (the first rak`ah)” (Ahmad, Muslim, Ibn Majah, and an-Nasa’i).