Doctrine & Covenants 121-123
These first two sections are fairly well known, and they've been quoted a lot in the recent general conferences. But I think Section 123 isn't as well-known. It contains, however, what I think might be my favorite phrase in the scriptures - "whole concatenation of diabolical rascality and nefarious and murderous impositions." Don't you love all those big words!!
It also contains the reminder that little things make great differences (a small helm moves a large ship) and that we should attend to things "with great earnestness" and "cheerfully do all things that lie in our power."
Whenever I'm struggling with that precept, I try to remember President Eyring's counsel from the October 2005 general conference:
"So, the great test of life is to see whether we will hearken to and obey God’s commands in the midst of the storms of life. It is not to endure storms, but to choose the right while they rage."
And he said basically the same thing the year before: "But the test a loving God has set before us is not to see if we can endure difficulty. It is to see if we can endure it well. We pass the test by showing that we remembered Him and the commandments He gave us. And to endure well is to keep those commandments whatever the opposition, whatever the temptation, and whatever the tumult around us."