And [mention, O Muhammad], when your Lord said to the angels, "Indeed, I will make upon the earth a successive authority."[1] They said, "Will You place upon it one who causes corruption therein and sheds blood, while we exalt You with praise and declare Your perfection?"[2] He [Allah] said, "Indeed, I know that which you do not know."

1- Khalifah: successor, or generations of man, one following another.

2- An additional meaning is "...and we purify ourselves for You."