We Got Work to Do
At the end of 1.01 Pilot, there is a shot of Dean and Sam standing over the trunk of the Impala. As Sam tosses a weapon into the trunk he says, "We got work to do" and closes the trunk. At the end of 2.22 All Hell Breaks Loose: Part Two, the scene is replayed with the brothers' positions reversed. Dean tosses the Colt into the trunk and says, "We got work to do." It is also re-created at the start of 10.05 Fan Fiction after Dean announces he has found a case. In 15.01 Back and to the Future, Sam says it and closes the trunk, while the image briefly flashes to the Winchesters closing the trunk at the end of the pilot.
In 1.12 Faith, when Dean asks Roy Le Grange what Le Grange saw in Dean that made him pick him out of the crowd to be healed, Le Grange replies, "A young man with an important purpose. A job to do. And it isn't finished." Other variations occur in 4.01 Lazarus Rising, when Dean asks Castiel why he was brought back and Castiel responds, "Because we have work for you," and in 7.19 Of Grave Importance, Bobby explains his continued presence as a ghost, saying "We still have work to do." In 13.05 Advanced Thanatology, after Billie reveals that she became the new Death, she tells Dean "You have work to do. That's all you need to know."
In 15.12 Galaxy Brain, Billie says to the boys "I told you Dean -- you and your brother have work to do. This is your destiny. You are the messengers of God's destruction."