Praise be to Allaah.
You should make a straight line. This is indicated by the hadeeth of al-Nu’maan ibn Basheer, who said: “The Prophet SAWS (peace and blessings of Allaah be upon him) said: “Make your rows straight, or Allaah will alter your faces.” [?] (Narrated by al-Bukhaari, al-Adhaan, 676). If the imaam and the one who is following him stand to pray, they should form one row, and the imaam should not stand ahead of the one who is following him. Because the Prophet SAWS (peace and blessings of Allaah be upon him) took hold of Ibn ‘Abbaas by the back of his head and made him stand to his right, but it was not narrated that he made him stand a little further back. The imaam and the one who is following him are considered to form a row, and if they are considered to form a row then it is prescribed for them to make the row straight, which means that neither of them should stand to the front of the other.
See al-Sharh al-Mumti’ by Ibn ‘Uthaymeen, vol. 3, p. 10-12
Imaam Ahmad narrated in his Musnad that Ibn ‘Abbaas said: “I came to the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) at the end of the night and prayed behind him. He took my hand and drew me to stand in line with him, and when the Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) turned back to his prayer, I took a step backwards. The Messenger of Allaah SAWS (peace and blessings of Allaah be upon him) prayed, and when he had finished his prayer, he said to me, ‘What is the matter? I made you stand in line with me, then you took a step backwards.’ I said, ‘ O Messenger of Allaah, is it right for anyone to pray in line with you when you are the Messenger of Allaah to whom Allaah has given …?’ (2902. Classed as saheeh by al-Albaani in al-Silsilah al-Saheehah, no. 2590). Al-Albaani (may Allaah have mercy on him) said: This hadeeth contains an important point of fiqh, which is that the Sunnah is for the one who is praying with the imaam to stand to his right and in line with him, not in front of him or behind him, unlike the view of some madhhabs which say that he should stand a little way behind the imaam, so that his toes are in line with the heel of the imaam. This, as you can see, goes against this saheeh hadeeeth.
See Silsilat al-Ahaadeeth al-Saheehah, vol. 6, p. 174. And Allaah knows best. May Allaah bless our Prophet Muhammad.