וְהָ֣אָדָ֔ם יָדַ֖ע אֶת־חַוָּ֣ה אִשְׁתּ֑וֹ וַתַּ֙הַר֙ וַתֵּ֣לֶד אֶת־קַ֔יִן וַתֹּ֕אמֶר קָנִ֥יתִי אִ֖ישׁ אֶת־ה׳׃
The man knew his wife, Chavah. She conceived and she gave birth to Kayin, and she said, I have obtained a man with Hashem.”
וַתַּהַר וַתֵּלֶד אֶת קַיִן
...וְרַבִּים בַּפָּרָשָׁה וּבִמְקוֹמוֹת אֲחֵרִים. אֶת ה׳ – עִם ה׳, כְּשֶׁבָּרָא אוֹתִי וְאֶת אִישִׁי הוּא לְבַדּוֹ בְּרָאָנוּ, אֲבָל בָּזֶה אָנוּ שֻׁתָּפִין עִמּוֹ, לְשׁוֹן רַשִׁ"י (רש"י על בראשית ד':א').
וְהַנָּכוֹן לִי שֶׁאָמְרָה הַבֵּן הַזֶּה יִהְיֶה לִי קִנְיָן לַה׳, כִּי כַּאֲשֶׁר נָמוּת יִהְיֶה בִּמְקוֹמֵנוּ לַעֲבֹד אֶת בּוֹרְאוֹ.
AND SHE CONCEIVED AND BORE CAIN.
“Eth Hashem (with the Eternal). When He created me and my husband He created us by Himself, but in the case of this one, we are co-partners with him.” Thus the words of Rashi.
The correct interpretation appears to me to be that she said: “This son will be for me an acquisition for the Eternal, for when we shall die he will exist in our stead to worship his Creator.”