"Eh! You seen one Earth, you've seen them all."
A little context for this one...
The Apollo 17 crew had arrived at the Moon and had just finished their first major task, getting the rover assembled, when Gene Cernan was distracted by the view of the Earth in the distance. He called to his partner, "Hey Jack, just stop. You owe yourself thirty seconds to look up over the South Massif at the Earth." "What? The Earth?" Jack replied. "Just look up there," Gene insisted, and this quote was Jack's reply as he went about his business doing some soil observations (I took a little artistic license by putting a rock in his hands, which made a better visual counterpoint to the Moon then him staring at the ground).
Gene notes in his book Last Man on the Moon that in that moment he was "almost disgusted with the blasé reaction" but I totally get it. They were already seven precious minutes behind schedule and with such limited time and an entire planet to explore, even 30 seconds wasted would mean one less sample collected or one less observation made. Besides that, Jack had spent most of the 3-day trip to the Moon looking back at the Earth working on a meteorology study, he had seen plenty of the Earth, this was his moment to see the Moon.
Upon his return to Earth Jack was quite amused to find that this quote had actually been picked up and co-opted by the environmental movement and could be found on a wide array of t-shirts and bumper stickers.
Countdown to Earth Day part 1 part 2