Supplement to the London Gazzette, January 1, 1923 (actually published on December 29, 1922). The name of Iqbal can seen on p.2., as a recipient of knighthood, and is written as ‘Dr. Sheikh Muhammad Ikbal, Barrister, Lahore, Punjab’.
Download the supplement in PDF from the official website, or go to p.2 (you can also navigate to other pages from there).
This is one of the references cited in Iqbal: Daur-i-Urooj, the fourth part of the comprehensive biography of Iqbal by Khurram Ali Shafique in Urdu.