Where was it first claimed that God is omnipotent or omniscient?
Did God or an authorized prophet ever actually make this claim?
Sources for G-d's omnipotence & omniscience are:
1) Divine omnipotence. From Wikipedia
The quote “Is anything too hard for the Lord?” is early and demonstrates omnipotence. (I would have thought that the creation of the world is sufficient demonstration.)
2) Divine omniscience. From Aish.com quoting the Rambam
G-d's knowledge of the evils demonstrates His omniscience.
3) For more detail see Derekh Hashem, “The Way of G-d” translated by Aryeh Kaplan, Chap 1, para 2, 4)
Omniscience and omnipotence are included in “ devoid of every conceivable deficiency”.
(Sources are Rambam Yad Yesodei HaTorah 2:8, Saadia Gaon Emunos vedeyos 1:4 and Joseph Albo, Ikkarim 2:1. You would have to look these up to find what they base themselves on.)