Analysis of information sources in references of the Wikipedia article "Sangam (1964 Hindi film)" in English language version.
most of the writers working in this so-called Hindi cinema write in Urdu: Gulzar or Rajinder Singh Bedi or Inder Raj Anand or Rahi Masoom Raza or Wajahat Mirza, who wrote dialogue for films like Mughal-e-Azam and Gunga Jumna and Mother India. So most dialogue-writers and most song-writers are from the Urdu discipline, even today.
most of the writers working in this so-called Hindi cinema write in Urdu: Gulzar or Rajinder Singh Bedi or Inder Raj Anand or Rahi Masoom Raza or Wajahat Mirza, who wrote dialogue for films like Mughal-e-Azam and Gunga Jumna and Mother India. So most dialogue-writers and most song-writers are from the Urdu discipline, even today.