Chapter on the Detailed Explanation of Prayer, Including the Obligatory and Recommended, and What is Permissible and Not Permissible.

And from him, from Muhammad ibn Ziyad, from Hammad ibn Uthman, from Udim ibn al-Hurr, who said: I heard Abu Abdillah عليه السلام say: "A man should not perform supererogatory (‘nafilah’) prayer once the time of an obligatory (‘faridah’) prayer has entered." And Imam عليه السلام said: "When the time of an obligatory prayer enters, begin with it."