Where is the most concrete exposition on the Buddhist ethics in the source material? In the Bible a complex saga of the human experience is elaborated in a multitude of ways, and these nuances make the text hard to crack in a linear reading. However, there exists a very illuminating chapter (Sermon on the Mount) that provides a very well articulated elaboration on what could be considered the core Christian ethics. It ultimately grounds the metaphysical landscape or at least informs many passages (both before and after it) to be more easily interpretted. 

I am interested in learning if there are any similar passages that could be pointed out in the Buddhist texts. 

My entirety of exposure to Buddhist source text currently is the Zen sutra on the lotus flower, and this question is open to any sect.
The Buddha taught for 45 years therefore his teachings are infinitely larger than those of Jesus. Therefore, there are so many (reported) discourses on ethics. Below are merely a few examples:


Magga-vibhanga Sutta: Noble Eightfold Path (for monks)
Cunda Kammaraputta Sutta: Ten Wholesome Actions (for laypeople) 
Veḷudvāreyya Sutta: The Golden Rule (for laypeople)
Sigalovada Sutta: The Layperson's Code of Discipline (in detail)
Vanijja Sutta: Right Livelihood (for laypeople)
Samajivina Sutta: Marriage (for laypeople)
Karaniya Metta Sutta: The Buddha's Words on Loving-Kindness


The above said, a fitting equivalent to the Sermon on the Mount, in terms of both content &amp; style, is the Maha-mangala Sutta, which similarly is about "Blessings". 

In addition, as the Sermon on the Mount was spoken 500 years after the Buddha, most of the topics in it would be found in the Buddha's teachings, such as: 


Integrity &amp; Dhamma having the taste of salt
Anger
Lust and sense control
Retaliation &amp; Love Your Enemies 
Spiritual treasure
Don't Judge Others
Crossing the floods with unshakable faith.


As for divorce, the Buddha taught life-long marriage is the ideal (Samajivina Sutta) yet it appears did not prohibit divorce when a wife is immoral.