In his commentary of Beresheith/Genesis 1:1, Rashi states:
And a further proof that the heavens and earth were not the first thing created is that the heavens were created from fire (אש) and water (מים), from which it follows that fire and water were in existence before the heavens
How did he come to this conclusion that the heavens were made from fire and water? What was his source?
As always scriptural references, articles, commentaries, and personal insight are always welcomed.