When the Jews are exiled to Babel, God commands Jeremiah to tell the Jews some instructions regarding their behavior in exile (Jeremiah 29:4-9):
"Thus said GOD of Hosts, the God of Israel, to the whole community that I exiled from Jerusalem to Babylon: Build houses and live in them, plant gardens and eat their fruit. You should take wives and beget sons and daughters; and you should take wives for your sons, and give your daughters to husbands, that they may bear sons and daughters. Multiply there, do not decrease. And seek the welfare of the city to which I have exiled you and pray to GOD in its behalf; for in its prosperity you shall prosper. For thus said GOD of Hosts, the God of Israel: Let not the prophets and diviners in your midst deceive you, and pay no heed to the dreams they dream. For they prophesy to you in My name falsely; I did not send them—declares GOD."
So this isn't from the Torah, and it isn't a Rabbinic mitzvah. What is it? Is it a sin if one transgresses it?
