Sayyiduna Jabir (may Allah be pleased with him) says: The Messenger of Allah (peace and blessings be upon him) came to us while we were raising our hands in prayer. He (peace and blessings be upon him) said: "Why do I see you raising your hands like the tails of restless horses? Be still in prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3521