This is a lovely song by Mukesh under Salil Chowdhary in an unreleased film by Mitti Ka Dev. The film's reels were destroyed in a fire. Hence it couldn't see the light of the day. The lyrics were by Gulzar.

As it happened, Gulzar later used the lyrics for a video song directed by Meghna Gulzar. The song, however, had a different tune. Song and music by Jagjit Singh.

In between, there is another version of the same lyrics, this time tuned by R D Burman, and sung by Asha Bhosle.

As it happened, the original Salil Chowdhary tune had a Bengali version too, and was sung by Mukesh. But apparently it didn't feature in any film (not sure of this part).

The story doesn't end here. Salil da used the same tune for a Hindi song for a never-made Bengali movie Ashwamedher Ghoda. Lyrics by Yogesh, & sung by KJ Yesudas.
Dunno whether any other tune & lyrics had gone through so many permutations & combinations.

PS: All info from YT comments section.

Had a wrong link for the Jagjit Singh song. Here is the right one.