Tamim bin Tarafah narrates from Sayyiduna Jabir bin Samurah (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) saw some people raising their hands in prayer, and he said: "They have raised their hands like the tails of unruly horses. Be still in prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب الجيم / 1811