It is mandatory to quote PAN on the return of income. Apart from return of income, PAN is also to be quoted in all challans for making payment of tax, correspondence with the Income Tax Department, etc.
However, w.e.f., 01-09-2019, every person who has been allotted a PAN, and who has linked his Aadhaar number with PAN as per section 139AA, may furnish his Aadhaar number in lieu of a PAN for all the transactions where quoting of PAN is mandatory as per Income-tax Act. Thus, with effect from September 1, 2019, an assessee can file his return of income by quoting his Aadhaar number instead of quoting PAN.
Source: Income Tax India’s Official Website