ליקוטי מוהר''ן חלק א' פרק ע''ב
The LeM of Rav Nahman of Breslov vol I chapter 72
Belief in God is rational. Everything has a cause. So unless there is a first cause, then you would have an infinite regress. And then nothing could exist. Therefore there must be a first cause. Therefore God, the first cause, exists. QED.