How to redirect the output of DBMS_OUTPUT.PUT_LINE to a file?

I need to debug in pl/sql to figure times of procedures, I want to use:

SELECT systimestamp FROM dual INTO time_db;
DBMS_OUTPUT.PUT_LINE('time before procedure ' || time_db);

but I don't understand where the output goes to and how can I redirect it to a log file that will contain all the data I want to collect?