It actually takes two commands.
I'll assume the good RAID-1 disk partition is /dev/hda1, the drive that was formatted is /dev/hdb, and the RAID-1 device is /dev/md0. As root:
1. Partition /dev/hdb the same as /dev/hda:
sfdisk -d /dev/hda | sfdisk /dev/hdb
2. Add the partition to the array:
mdadm /dev/md0 --add /dev/hdb1
You can verify that it is syncing up with: