The right side of the row is better than the left

If only one man is praying with the imam, then he should stand to the imam’s right, and not on his left, because of the hadeeth of Ibn ‘Abbas (may Allah be pleased with him), who stayed overnight with his maternal aunt Maymoonah (may Allah be pleased with her); the Prophet (peace and blessings of Allah be upon him) got up to pray qiyaam al-layl, and Ibn ‘Abbas stood on his left, and he reached behind him and made him stand on his right. This indicates that if only one person is praying behind the imam, he should stand on the right and not on the left. 

But if more than one person is praying behind the imam, then they should stand behind him, and the right side of the row is better than the left. This applies if the two sides of the row are similar, but if the right side of the row is clearly stretching further than the left, then being on the left and closer to the imam is better.  

Based on this, the people praying behind the imam should not all be on the imam’s right to the extent that there is no one on his left except one or two men. That is because what is prescribed in the case of three men is that the imam should be between them, with one man on his right and one on his left; they should not all be on his right. This indicates that the imam should be in the middle of the row or thereabouts. 

To sum up: the right is better if the sides are equal or close to that. But if the right hand side stretches further, then the left is better, because it is closer to the imam."(Majmoo’ Fatawa al-Shaykh Ibn ‘Uthaymeen  12/184)