Musnad Ahmad 907:152

Yahya ibn Sa'id narrated to us, from al-A'mash, who said: Musayyib ibn Rafi' narrated to me, from Tamim ibn Tarfah, from Jabir ibn Samurah, that the Messenger of Allah (may Allah bless him and grant him peace) entered the mosque while they were in circles, and he said: "What is the matter with me that I see you in groups?" And the Messenger of Allah (may Allah bless him and grant him peace) entered the mosque while they had raised their hands, and he said: "They have raised them as if they were the tails of restive horses. Be tranquil in prayer."