1) You can use report RPR_ABAP_SOURCE_SCAN.
2) Old method is to use report RSRSCAN1. Use with the Z* in the program parameter to search in custom programs only.
Otherwise, it will take a long time to search. Enter the string to-be-searched in "Find string parameter".
3) Try transaction EWK1.
If you already know program name:
Then use the function "Find in source code" in the ABAP Editor instead.
(Transaction SE38, Menu "Utilities" -> Find in Source Code) or Use program SAPLS_SEARCH
