Many people seem to think that the song Ae dil mujhe aisi jagha le chal jahan koi na ho, sung by him for Anil Biswas for the movie Arzoo(1950) was his first recording for a Hindi movie. No doubt it was very popular but its not his first song in hindi films.
Some unconfirmed reports, say, that the lovely Sundarta ke Sabhi Pujaari which he sang for Bulo C Rani for Jogan(1950) may have been recorded earlier.
The real fact is that he got an opportunity to sing in Hindi films during his stay in Calcutta itself. He sang as many as two songs for the Kanan Devi starrer, Rajlaxmi in 1945.
The first song in the movie was Jaago Musafir Jaago Kholo Man Ka Dwar
The second song in the movie was Tu Sun Le Matwaale
His first non-film hindi song, of course, was way back in 1941 for HMV, sab din ek samaan nahin tha, ban jaoonga kya se kya main, iska to kuch dhyan nahin tha.