The phrase "thus says the Lord" appears throughout the book of Exodus as a way for God to get the attention of people who really need his help, whether due to their oppressive circumstances or their extensive sin and rebellion.
The Minor Prophets pick up on these twin themes of God's unending care and God's redeeming judgment. Amos 1:3 states, "Thus says the LORD: 'For three transgressions of Damascus, and for four, I will not turn away its punishment, because they have threshed Gilead with implements of iron'" (NKJV). The book of Amos is primarily about God's relationship with the Northern Kingdom of Israel - or, really, lack of one. But in the course of addressing Israel, God makes it clear that he is concerned about the nations that surround Israel. Damascus was the ancient capital of the Kingdom of Aram, which was immediately north of Israel.
Aram could be a tricky neighbor; while they would occasionally ally with Israel, they usually pillaged from her. And this is exactly God's problem with Aram: they harassed and took over the agricultural riches of Gilead, which was a portion of northern Israel rich in good farming and grazing land. However, God's problem with Aram (represented by its capital Damascus) wasn't so much THAT they took from Israel, but HOW they took from Israel: "they have threshed Gilead with implements of iron."
This is a very violent image of harvesting. Instead of gleaning crops, God points out that Aram culled the people of Gilead - most likely a reference to genocide. I'm not sure how the ancient world would have categorized this sin, but we would identify genocide as a human rights violation. According to Amos, God cares about the rights of the people he created, and he shows no hesitancy in speaking to the oppressor about his sin or the victim about his loss.
Friends, God's "coh amar y'adonai" - thus says the Lord - is intended to nurture human thriving by freeing us from genocidal tendencies and practices, because of the simple fact that "thus says the Lord."