First redirect stderr to stdout — the pipe; then redirect stdout to
/dev/null (without changing where stderr is going):
command 2>&1 >/dev/null | grep 'something'
For the details of I/O redirection in all its variety, see the chapter on Redirections in the Bash reference manual.
Note that the sequence of I/O redirections is interpreted left-to-right, but pipes are set up before the I/O redirections are interpreted. File descriptors such as 1 and 2 are references to open file descriptions. The operation
2>&1 makes file descriptor 2 aka stderr refer to the same open file description as file descriptor 1 aka stdout is currently referring to (see
open()). The operation
>/dev/null then changes file descriptor 1 so that it refers to an open file description for
/dev/null, but that doesn’t change the fact that file descriptor 2 refers to the open file description which file descriptor 1 was originally pointing to — namely, the pipe.