Musnad Ahmad 30:987

Yahya ibn Adam narrated to us, Sharik narrated to us, from Yazid ibn Abi Ziyad, from Mujahid, from Abu Hurairah, who said: The Messenger of Allah, may Allah bless him and grant him peace, commanded me with three things and forbade me from three things: He commanded me with two rak'ahs of Duha every day, the Witr before sleep, and fasting three days of every month. And he forbade me from the pecking like the pecking of a rooster, the sitting (iq'a) like the sitting of a dog, and the turning (looking around) like the turning of a fox.