Musnad Ahmad 907:158

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Musayyib ibn Rafi', from Tamim ibn Tarafah, from Jabir ibn Samurah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) came out to us one day and said: "What is the matter with me that I see you raising your hands as if they were the tails of restive horses? Be still in prayer." Then he came out to us and saw us in circles, and said: "What is the matter with me that I see you in separate groups?" Then he came out to us and said: "Will you not line up as the angels line up before their Lord?" They said: "O Messenger of Allah, and how do the angels line up before their Lord?" He said: "They complete the first rows, and they stand close together in the row."