All of us know about non-film songs that were used in films. These are from private records, dramas, nautankis etc. Perhaps the most famous is "nadee naare na jaiyo shaam" originally by Gulab Bai, the famous nautanki artist. It was used in Mujhe Jeene Do (1961).
But which was the earliest song so used? To answer that we would need to know about non-film records. Luckily, MIchael Kinnear of the SIRC has done all the work. From his first book that covers 1899-1980, we find the song "jo main aisaa jaanatee preet kiye dukh hoye" from the play Bhool Bhulaiyan, sung by Miss Subhashi. Interestingly, a talkie was made in 1933 with the same name, most likely based on this drama, but this song was not used.
Of course, everybody knows at least one Hindi film in which this song was used.