Question

the source of authority of the indian constitution is

Answer 1

The source of authority of the Indian constitution is a supreme judicial authority.

The primary line of Indian Constitution is: 
"We the people of India" 
Which implies that individuals of India are themselves the INDIAN CONSTITUTION 
The Constitution is an extreme blessing by the general population of India, for The general population of India.It's not Parliment, President, PM, but rather The People of India who are Sovereign.
Albeit constituent gets together of India was not specifically chosen body but rather the vast majority of the gathering individuals are mass pioneers, spoke to the voice of a large number of people groups, had wide acknowledgment Constituent Assembly speak to the voice of all segments of India Society:
i.e Women ,minority ,SC/ST 
So, even Parliment can't change constitution at its impulses (it is subjected to essential structure regulation led if there should be an occurrence of Keshvanand Bharati versus condition of Kerala)

Answer 2

The Preamble is a short introductory statement to the Constitution of India that denotes the purpose and principles, it also indicates the source from which it derives its authority, the people. 75% of the Indian constitution has been adopted from the Articles of Government of India Act, 1935 Act. So, Indians or more precisely , the People of India are themselves authority of The Indian constitution. It is ultimate gift by the people of India ,for the people of India .