Musnad Ahmad 907:221

Waki’ narrated to us, Al-A’mash narrated to us, from Al-Musayyab ibn Rafi’, from Tamim ibn Tarfah, from Jabir ibn Samurah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) entered upon us while we were raising our hands in prayer. He said: "What is the matter with me seeing you raising your hands as if they were the tails of restive horses? Be still in prayer." He said: And he entered the mosque upon us while we were in separate circles, and he said: "What is the matter with me seeing you in separate groups?"