Furthermore, when you've finished, the user should be able to connect using the same password.
Each user's password is encrypted and stored in the database in the SYS.USER$ table, under PASSWORD column and in 11g databases this is the only place you can find it, since it was removed from DBA_USERS view [Oracle note 735651.1].
You may query the DBA_USERS to get it:
SQL> SELECT PASSWORD FROM DBA_USERS WHERE USERNAME='AARGY'; PASSWORD ---------------- 5161D15AA3324F0A
Now, change user's password, so you may connect to the database using that account:
SQL> ALTER USER AARGY IDENTIFIED BY TEST_123; User altered. SQL> CONN AARGY/TEST_123 Connected.
You've finished and it's time to restore the password, so:
SQL> ALTER USER AARGY IDENTIFIED BY VALUES '5161D15AA3324F0A'; User altered. SQL> CONNECT AARGY/TEST_123 ERROR: ORA-01017: invalid username/password; logon denied