Musnad Ahmad 32:1953

Yazid ibn Harun narrated to us, Shu'bah informed us, Qatadah informed us, from Anas, from the Prophet (peace and blessings of Allah be upon him), who said: "Maintain balance in prayer, and let not one of you stretch out his forearms like the stretching out of a dog." Thus said Yazid: Maintain balance in prayer.