People praying behind the Imaam should stand to his right and to his left
10-4-2014 | IslamWeb
Question:
Regarding the first line for salah should we start and finish the right of the line before we start the left side?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
said: “Place the Imaam in the middle (of the row) and fill the gaps.” [Abu Daawood].”
The wisdom for the Imaam standing in the middle, as the scholars said, is to let everyone hear him, those on the right as well as those on the left, and also, so that everyone follows him, those on the right and those on the left. If he were at one end, then it is only those on that end who would see him and hear him.
Therefore, the people praying (behind the Imaam) should start standing at the right of the Imaam and at his left in a way that the Imaam would be in the middle. But there is no doubt that hastening to be on the right is better as the Prophet said: “Allaah and His Angels invoke blessings upon those who are standing at the right side of the rows.” [Abu Daawood and Ibn Maajah] Ibn Hajar said that its chain of narrators is Hasan (good).
Also, Al-Baraa’ 
said: “When we prayed with the Prophet we liked to be on his right.” [An-Nasaa'i] Ibn Hajar said that its chain of narrators is Saheeh (sound).
However, the preference of the right side is not absolute in all cases; rather, in case the right side is nearly the same as the left side. But if there is a big difference between them, when the right side is longer, then standing on the left side of the Imaam is better in this case.
Shaykh Ibn Uthaymeen 
was asked: “If the right side of the row is longer than the left side, where should one pray, on the right side or the left side?” He replied: “If the difference is big, then pray on the left. But if they are equal or the difference is slight, then pray on the right.” He was asked: “Should the Imaam order them to move when there is a big difference?" He replied: “Yes, we order them to do that; from the right to the left and vice versa.” [End of quote]
Besides, Majmoo' Al-Fataawa of Shaykh Ibn Uthaymeen reads: “The right side of the row is better than the left side. This is if they are nearly of the same length, but if the right side continues to get longer, then being on the left side and closer to the Imaam is better. Therefore, the people led in prayer should not be on the right side of the Imaam to an extent that there is only one man or two on the left side. This is because when there are only three men, the Imaam stands right between them in the front, in a way that one would be somewhat to his right side and the other to his left, and they do not both stand to his right. This is evidence that the Imaam stands in the middle of the row or nearly so. In brief, the right side is better if the rows are equal or nearly equal, but if the right side is too long, then the left side is better because it is closer to the Imaam.” [End of quote]
Therefore, dear Brother, you should urge the worshipers to stand to the right and left of the Imaam so that he would be in the middle of the row or nearly so, and the one who guides people to what is good is like the one who does it.
For more benefit, please refer to Fatwa 90164.
Allaah Knows best.