MHH 12 and 13

A Memorable Fancy.
The Prophets Isaiah and Ezekiel dined with me, and I asked them how they dared so roundly to assert that God spake to them; and whether they did not think at the time, that they would be misunderstood, & so be the cause of imposition.
Isaiah answer'd. 'I saw no God, nor heard any, in a finite organical perception; but my senses discover'd the infinite in every thing, and as I was then perswaded, & remain confirm'd, that the voice of honest indignation is the voice of God, I cared not for consequences but wrote.'
Then I asked: 'does a firm perswasion that a thing is so, make it so?'
He replied: 'All poets believe that it does, & in ages of imagination this firm perswasion removed mountains; but many are not capable of a firm perswasion of any thing.'
Then Ezekiel said. 'The philosophy of the east taught the first principles of human perception: some nations held one principle for the origin & some another; we of Israel taught that the Poetic Genius (as you now call it) was the first principle and all the others merely derivative, which was the cause of our despising the Priests & Philosophers of other countries, and prophecying that all Gods would ....

It's interesting how these two plates ran together.  In later illuminated works he often did that. 

The words he wrote here might be considered additions to Isaiah and Ezekiel, although I haven't found whatever places Blake may have used as a 'source'.

at last be proved to originate in ours & to be the tributaries of the Poetic Genius; it was this that our great poet King David desired so fervently & invokes so pathetic'ly, saying by this he conquers enemies & governs kingdoms; and we so loved our God. that we cursed in his name all the deities of surrounding nations, and asserted that they had rebelled; from these opinions the vulgar came to think that all nations would at last be subject to the jews.'
'This' said he, 'like all firm perswasions, is come to pass; for all nations believe the jews' code and worship the jews' god, and what greater subjection can be?'
I heard this with some wonder, & must confess my own conviction. After dinner I ask'd Isaiah to favour the world with his lost works; he said none of equal value was lost. Ezekiel said the same of his.
I also asked Isaiah what made him go naked and barefoot three years? he answer'd, 'the same that made our friend Diogenes the Grecian.'
I then asked Ezekiel why he eat dung, & lay so long on his right & left side? he answer'd, 'the desire of raising other men into a perception of the infinite; this the North American tribes practise, & is he honest who resists his genius or conscience. only for the sake of present ease or gratification?'

Here we can find 'sources': Isaiah wrote that God had told him to go naked and barefoot in Chapter 20. 

Re Ezekiel laying on his right and left side so long, look at Ezekiel 4:4-6. 

Singer discussed these Plates in  pages 113-24 of The Unholy Bible:
She seemed to suggest that Blake's mind moved out of commonplace material events, out of the world so to speak, and that he learned to clothe his 'transcendent' thoughts in terms of biblical figures, especially the prophets.

Marriage of Heaven and Hell may have been Blake's first attempt to express to others these feelings. 

A Memorable Fancy begins with Blake's statement that the Prophets Isaiah and Ezekiel dined with him, which is evocative of Revelation 3:2:
"Behold, I stand at the door, and knock: if any man hear my voice, and open the door, I will come in to him, and will sup with him, and he with me."

Blake's Ezekiel wrote, "the Poetic Genius (as you now call it) was the first principle".  See also All Religions are One (Erdman 1)