Held, that issue of notice under section 143(2) of the Act within the statutory period before completion of assessment was mandatory in nature. The Revenue authorities were not in a position to show that there was a valid service of notice under section 143(2) before completion of assessment. Thus, the assessment order was void ab initio and did not survive. Where there was no valid assessment order that order could not be the subject matter of revision under section 263 of the Act by the Principal Commissioner. Therefore, the revision order was to be quashed.(AY. 2017-18).