create or replace trigger acc_det_update_trigger before update on
account_details for each row
begin
insert into acc_det_backup select * from acc_det where Comp_acct in
(select comp_acct from acc_det where comp_acct =ld.comp_acct );
end ;
/

Hear I want to insert the rows into the backup table before the rows are updated .I am getting these errors

ERROR at line 1:
ORA-04091: table TESTUSER.ACC_DET is mutating, trigger/function may not see it
ORA-06512: at "TESTUSER.ACC_DET_UPDATE_TRIGGER", line 2
ORA-04088: error during execution of trigger 'TESTUSER.ACC_DET_UPDATE_TRIGGER'