The first prohibition is that we are forbidden from entertaining the thought1 that there is any deity other than G‑d.
The source of this prohibition is G‑d's 'statement'2 (although He is so exalted that the term "speech" cannot really be applied to Him), "Do not have any other gods before me."
In the end of Makkos3 it is explained that this prohibition is included among the 613 commandments, as they state there, "613 commandments were given to Moshe at Sinai." We have explained this proof in the 1st of the Positive Mitzvos.
