What is the halachic source for the minhagim of mourning during Sefira?

Question

What is the earliest halachic source for the minhagim of mourning during sefira? Note that this does not mean the story of R' Akiva, I'm talking about halachic references to prohibitions such as not cutting hair, not listening to music etc.

Answer 1

The Beit Yosef (OC 493) quotes Rabbeinu Yerocham (~1350 CE) who quotes "Geonim" who mention a custom not to get married between Pesach and Shavuot.

Answer 2

As DoubleAA pointed out, the Beis Yosef quotes the "Gaonim".

Those "Gaonim" could be referring to the Halachos Pesukos, which was a sefer written by one of the Gaonim, possible Rabbi Yehudai Gaon.

He writes:

וששאלתם למה אין מקדשין ואין כונסין בין פסח לעצרת אם מחמת איסור ואם לאו הוו ידעין שלא משום איסור נגעו בה אלא משום מנהג אבילות שכך אמרו חז"ל שנים עשר אלפים זוגים תלמידים היו לו לרי עקיבא וכולן מתו בין פסח לעצרת על שלא נהגו כבוד זה בזה. ותאני עלה וכולן מתו מיתה משונה באסכרה ומאותה שעה ואילך נהגו ראשונים בימים אלו שלא לכנוס בהן ומי שקפץ וכנס קונסין לא עונש ולא מלקות אבל אם בא לשאול לכתחילה אין אנו אין מורים לו לכנוס ולענין קידושין מי שרצה לקדש בין פסח לעצרת מקדש לפי שאין עיקר שמחה אלא בחופה

You ask why don't we marry between Pesach and Shavuos - is it because it's prohibited by law?

You should know that the reason is because of the custom of mourning - as the Rabbis taught that Rabbi Akiva had twelve-thousand pairs of students, and they all died between Pesach and Shavuos because they didn't honor each other, and they all died a horrible death (askara).

Since then, the custom was not to get married during that period. If someone did get married, we don't fine him, but if he comes and asks, we don't tell him to get married. However, one is allowed to do Kiddushin (the first stage of marriage), as the main joy is the Chuppah.

Answer 3

In addition to the answer from Double AA, the Beit Yosef goes on to say ויש מקומות שנהגו שלא להסתפר .{There are places where the custom is not to have a haircut} during this period. Mishna Berura siman 493(3) says one may make a festive meal for an engagement but not to make ריקודין ומחולות {dancing}. Rabbi Mansour writes that the Magen Abraham adds that one should also refrain from listening to music during the Omer period. Although the Shulhan Aruch makes no mention of this practice, Jews have accepted this custom and one should therefore not listen to music during the first thirty-three days of the Omer.