Sahih Muslim Book 4, Hadith Number 864.

Jabir b. Samura reported: The Messenger of Allah (may peace be upon him) came to us and said: How is it that I see you lifting your hands like the tails of headstrong horses? Be calm in prayer. He (the narrator) said: He then again came to us and saw us (sitting) in circles; he said: How is it that I see you in separate groups? He (the narrator) said: He again came to us and said: Why don’t you draw yourselves up in rows as angels do in the presence of their Lord? We said: Messenger of Allah, bow do the angels draw themselves up in rows in the presence of their Lord? He (the Holy Prophet) said: They make the first rows complete and keep close together in the row.