I am aware of discussions in the Gemara and Rishonim of the permissibility of various methods of birth control.
What I am looking for is the earliest mention of the modern day practice in which a couple visits a Rav to ask permission to use birth control at all, whether as newlyweds or after already having had some children.
While I'm primarily interested in the earliest mention of the expectation that a couple consult with a Rav for their individual circumstance each time they want permission to use birth control, I'm also interested in sources discussing the permissibility of various timings of birth control in general. (I'm familiar with multiple modern day blanket rulings such as "it is permissible to space between children X amount" (where X varies by the Rav giving the psak) or permission to use birth control right after the wedding for 3 months versus not at all. I haven't yet been able to find equivalent discussions in less modern sources.)