How a country is categorized as developed or developing? Can you share an official source or reference of definition of a developed and developing country? some IMF or World Bank definition with link would be preferred. I need to write the reference in my thesis. Thanks
The terms developed and developing are UNDP terms.

Read the paper at the link below - it goes through the UNDP, World bank and IMF classifications and it's an IMF working paper so i'd class that as referenceable. At the very least it will point you to better sources.

https://www.imf.org/external/pubs/ft/wp/2011/wp1131.pdf