Perhaps you’re reffering to training in sila? Because sila and metta goes together and are somewhat inseparable, as could be implied from this post by Bhante @Dhammanando in this thread:
In gradual training of buddhism, in general indeed it starts from sila practice. DN2 is probably most compherensive on subject of gradual training, and first section is clearly on ethics and livelihood.
DN2: The Fruits of the Ascetic Life
4.3. The Finer Fruits of the Ascetic Life
“But sir, can you point out a fruit of the ascetic life that’s apparent in the present life which is better and finer than these?”
“I can, great king. Well then, listen and pay close attention, I will speak.”
“Yes, sir,” replied the king.
The Buddha said this:
Bhagavā etadavoca:
“Consider when a Realized One arises in the world, perfected, a fully awakened Buddha, accomplished in knowledge and conduct, holy, knower of the world, supreme guide for those who wish to train, teacher of gods and humans, awakened, blessed. He has realized with his own insight this world—with its gods, Māras and Brahmās, this population with its ascetics and brahmins, gods and humans—and he makes it known to others. He teaches Dhamma that’s good in the beginning, good in the middle, and good in the end, meaningful and well-phrased. And he reveals a spiritual practice that’s entirely full and pure.
A householder hears that teaching, or a householder’s child, or someone reborn in some clan. They gain faith in the Realized One, and reflect: ‘Living in a house is cramped and dirty, but the life of one gone forth is wide open. It’s not easy for someone living at home to lead the spiritual life utterly full and pure, like a polished shell. Why don’t I shave off my hair and beard, dress in ocher robes, and go forth from the lay life to homelessness?’
After some time they give up a large or small fortune, and a large or small family circle. They shave off hair and beard, dress in ocher robes, and go forth from the lay life to homelessness.
Once they’ve gone forth, they live restrained in the monastic code, conducting themselves well and seeking alms in suitable places. Seeing danger in the slightest fault, they keep the rules they’ve undertaken. They act skillfully by body and speech. They’re purified in livelihood and accomplished in ethical conduct. They guard the sense doors, have mindfulness and situational awareness, and are content.
4.3.1. Ethics
4.3.1.1. The Shorter Section on Ethics
And how, great king, is a mendicant accomplished in ethics? It’s when a mendicant gives up killing living creatures, renouncing the rod and the sword. They’re scrupulous and kind, living full of compassion for all living beings. This pertains to their ethics.
They give up stealing. They take only what’s given, and expect only what’s given. They keep themselves clean by not thieving. This pertains to their ethics.
They give up unchastity. They are celibate, set apart, avoiding the common practice of sex. This pertains to their ethics.
They give up lying. They speak the truth and stick to the truth. They’re honest and trustworthy, and don’t trick the world with their words. This pertains to their ethics.
They give up divisive speech. They don’t repeat in one place what they heard in another so as to divide people against each other. Instead, they reconcile those who are divided, supporting unity, delighting in harmony, loving harmony, speaking words that promote harmony. This pertains to their ethics.
They give up harsh speech. They speak in a way that’s mellow, pleasing to the ear, lovely, going to the heart, polite, likable and agreeable to the people. This pertains to their ethics.
They give up talking nonsense. Their words are timely, true, and meaningful, in line with the teaching and training. They say things at the right time which are valuable, reasonable, succinct, and beneficial. This pertains to their ethics.
They refrain from injuring plants and seeds. They eat in one part of the day, abstaining from eating at night and food at the wrong time. They avoid dancing, singing, music, and seeing shows. They refrain from beautifying and adorning themselves with garlands, fragrance, and makeup. They avoid high and luxurious beds. They avoid receiving gold and money, raw grains, raw meat, women and girls, male and female bondservants, goats and sheep, chickens and pigs, elephants, cows, horses, and mares, and fields and land. They refrain from running errands and messages; buying and selling; falsifying weights, metals, or measures; bribery, fraud, cheating, and duplicity; mutilation, murder, abduction, banditry, plunder, and violence. This pertains to their ethics.
With Metta 
