Musnad Ahmad 30:477

Muhammad ibn Fudayl narrated to us, Yazid ibn Abi Ziyad narrated to us, he who heard Abu Hurayrah say narrated to me: My intimate friend [the Prophet] advised me with three things and forbade me from three things: "He advised me to perform the Witr prayer before sleeping, to fast three days of every month, and to perform the two rak'ahs of Duha." He said: "And he forbade me from looking around [during prayer], from sitting like the sitting of a monkey, and from pecking like the pecking of a rooster."