How do we know that God said 'Let there be light'?

Answer 1

Genesis 1:3: "And God said, Let there be light: and there was light." To a biblical literalist, this is all the evidence we need that God spoke these exact words.

However, it is now known that Genesis chapter 1 was actually written quite late in Jewish history - during the Babylonian Exile. This is a very poetic statement of creation, but there is no reason either to believe it was divinely inspired or that these were the very words of God.

For more information, please visit: http://christianity.answers.com/theology/the-story-of-creation

Answer 2

The author of the Torah, in its entirety, was Moses (Deuteronomy 31:24), to whom God dictated its text (Exodus 24:12), including Genesis.Note also, that Genesis did not come to Moses as a surprise. The Israelites had always handed down the earliest traditions, even before Moses:What_did_the_early_israelites_read

According to tradition, there is only one Genesis creation-narrative, with ch.2 serving as an expansion of the brevity of ch.1, not a separate set of events (Rashi commentary, Gen.2:8). In ch.1, God created the universe from nothing (Exodus 20:11, Isaiah 40:28; Maimonides' "Guide," 2:30; Targum and Nachmanides on Gen. 1:1; Rashi commentary, Gen.1:14), and in ch.2, God performed specific acts within the broader picture.
While Judaism has always seen the Torah as an intricate tapestry that nonetheless had one Divine source, some modern authors such as Wellhausen (the father of modern Biblical-criticism, 1844-1918) have suggested artificially chopping up the narrative and attributing it to various authors, despite the Torah's explicit statement as to its provenance (Exodus 24:12, Deuteronomy 31:24). This need not concern believers, since his claims have been debunked one by one, as Archaeology and other disciplines have demonstrated the integrity of the Torah. No fragments have ever been found that would support his Documentary Hypothesis, which remains nothing more than an arbitrary claim:

http://religion.answers.com/theory/debunking-the-jepd-documentary-hypothesis

http://www.apologeticspress.org/apcontent.aspx?category=6&article=1131(a Christian author)