Where was it first claimed that God is omnipotent or omniscient?
Did God or an authorized prophet ever actually make this claim?
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Sources for G-d's omnipotence & omniscience are:
1) Divine omnipotence. From Wikipedia
The Jewish belief in God's omnipotence is rooted in the Bible:
'Why did Sarah laugh, and say "Shall I indeed bear a child now that I am old?" Is anything too hard for the Lord?'"[Gen. 18:13-3] “Attribute to the Lord all glory and power.”[Ps. 29]
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
The Almighty knows the actions of people and does not ignore them. [It is] not like those who say (Ezekiel 8:12), "God has abandoned the land," but rather like [the Scriptures that attest to His scrutiny], as it says (Genesis 6:5), "The Almighty saw that there was great evil perpetrated by man in the land..." and (Genesis 18:20), "The Almighty said, 'The cry of Sodom and Gemorrah is great...' " All this attests to this principle.
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)
The only thing that we can know about Him is that he is perfect in every possible way and devoid of every conceivable deficiency.
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.)