Musnad Ahmad 907:166

Muhammad ibn Ubayd narrated to us, Mis'ar narrated to us, from Ubayd Allah ibn al-Qibtiyyah, who said: I heard Jabir ibn Samurah say: We used to say behind the Messenger of Allah, peace and blessings of Allah be upon him, when we performed the taslim: "As-salamu alaykum, as-salamu alaykum," and one of us would gesture with his hand to his right and to his left. The Messenger of Allah, peace and blessings of Allah be upon him, said: "What is the matter with those who gesture with their hands during prayer as if they were the tails of restive horses? Is it not sufficient for one of you to place his hand on his thigh, then say the taslim to his right and to his left?"