Adam knew his wife, Eve, resulting in the conception of their first child, Cain, as described in Genesis 4:1.
Genesis 4:1 provides the direct answer to the question: "And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD." This verse indicates that Adam had sexual relations with Eve, his wife, after which she conceived and gave birth to Cain. The Bible doesn't provide an exact date or time, but it clearly states that this occurred after their creation and placement in the Garden of Eden (as described in Genesis 2 and 3) and after their expulsion from the garden as a consequence of eating from the Tree of the Knowledge of Good and Evil.
It is important to note that the phrase "knew his wife" is a euphemism used in the Bible to describe sexual intimacy. This phrasing is common in the Old Testament and is a culturally sensitive way of describing the act of procreation.
Therefore, the answer is found directly in Genesis 4:1.
