Yoga sutra number 33, as discovered in Neal Pollack’s essay on being happy for friends’ success instead of envious:
In daily life we see people around who are happier than we are, people who are less happy. Some may be doing praiseworthy things and others causing problems. Whatever may be our usual attitude toward such people and their actions, if we can be pleased with others who are happier than ourselves, compassionate toward those who are unhappy, joyful with those doing praiseworthy things, and remain undisturbed by the errors of others, our mind will be very tranquil.
I love this. If I had already dug the printer out of the garage, I would print this and hang it on the wall.