A sad violin song in most of flashback scenes like Obito's flashback and the storybook from Jiraiya
This was not released in any of the official OSTs. At the very least, it doesn't appear in any of the tracks in this list (as of 2018).

There are some unofficial versions online, such as this one, titled "Minato saves Kushina".

I remember this song as that nostalgic-sounding track that is mostly happy, but has just a touch of bittersweetness, and also that really good violin bit.

Not to be confused with "Let Me Tell You a Story from My Childhood", which sounds similar and also has some very nice violin parts, but unlike the first song, is more lighthearted and cheerful.