Section 4(2)(b) in The Aadhaar (Authentication) Regulations, 2016
(b)One-time pin based authentication: A One Time Pin (OTP), with limited time validity, is sent to the mobile number and/ or e-mail address of the Aadhaar number holder registered with the Authority, or generated by other appropriate means. The Aadhaar number holder shall provide this OTP along with his Aadhaar number during authentication and the same shall be matched with the OTP generated by the Authority.