If you are supplying only exempted goods, then you don't have to obtain GSTIN irrespective of the fact that your turnover exceeds Rs.40 Lakhs. But if you supply taxable goods also along with exempted goods, then you have to obtain GSTIN.
Check Section 23 of the CGST Act,2017 for reference.
Dear Hemanth, no specific notification but its specified in section 23(1)(a) any person engaged exclusively in the business of supplying goods or services or both that are not liable to tax or wholly exempt from tax under this Act. Therefore he need not obtain registration. see CBIC faq- https://cbec-gst.gov.in/faq.H T M L