I have tried :
RMAN> backup database with differential;

RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-00558: error encountered while parsing input commands
RMAN-01005: syntax error: found "identifier": expecting one of: "archivelog, backup, backupset, channel, controlfilecopy, current, database, datafile, datafilecopy, delete, diskratio, filesperset, format, force, include, keep, (, maxsetsize, noexclude, nokeep, not, parms, pool, plus, ;, skip, spfile, setsize, tablespace, tag"
RMAN-01008: the bad identifier was: with
RMAN-01007: at line 1 column 17 file: standard input

WITH is not correct.

Does anyone knows the syntax of this command?