When in the history, did people begin to have 8 threads in Tzizit?

Question

Rambam (Tzizit 1;1) says that "No definite number of threads is prescribed in the Torah for the fringes".

Then my question is: When did the tradition of having 8 threads start?

Answer 1

In Menachot 41b we have the following dispute:

ת"ר כמה חוטין הוא נותן ב"ש אומרים ד' וב"ה אומרים ג

Our Rabbis taught: How many threads must one insert? Beth Shammai say, Four; but Beth Hillel say. Three.

(Soncino translation)

This tells us that in tannaic times there was a dispute as to the number of required strings. As noted by Tosafot there, Scriptural support for the view requiring four strings is provided on 39b.

In Bava Batra 73b the Talmud tells the tale of Rabba Bar Bar Chana attempting to see if the dead bodies in the wilderness had the number of strings in accordance with Beit Shammai or Beit Hillel:

ההוא טייעא דהוה שקיל עפרא ומורח ליה ואמר הא אורחא לדוכתא פלן והא אורחא לדוכתא פלן אמרי' ליה כמה מרחקינן ממיא ואמר לן הבו לי עפרא יהיבנן ליה ואמר לן תמני פרסי תנינן ויהבינן ליה אמר לן דמרחקינן תלתא פרסי אפכית ליה ולא יכילית ליה אמר לי תא אחוי לך מתי מדבר אזלי חזיתינהו ודמו כמאן דמיבסמי וגנו אפרקיד והוה זקיפא ברכיה דחד מינייהו ועייל טייעא תותי ברכיה כי רכיב גמלא וזקיפא רומחיה ולא נגע ביה פסקי חדא קרנא דתכלתא דחד מינייהו ולא הוה מסתגי לן אמר לי דלמא שקלת מידי מינייהו אהדריה דגמירי דמאן דשקיל מידי מינייהו לא מסתגי ליה אזלי אהדרתיה והדר מסתגי לן כי אתאי לקמיה דרבנן אמרו לי כל אבא חמרא וכל בר בר חנה סיכסא למאי הלכתא עבדת הכי למידע אי כבית שמאי אי כבית הלל איבעי לך למימני חוטין ולמימני חוליות

Rabbah b. Bar Hana related: We were once travelling in a desert and there joined us an Arab merchant who, [by] taking up sand and smelling it [could] tell which was the way to one place and which was the way to another. We said unto him: 'How far are we from water?' He replied: 'Give me [some] sand.' We gave him, and he said unto us: 'Eight parasangs.' When we gave him again [later], he told us that we were three parasangs off. I changed it; but was unable [to nonplus] him.

He said unto me: 'Come and I will show you the Dead of the Wilderness.' I went [with him] and saw them; and they looked as if in a state of exhilaration. They slept on their backs; and the knee of one of them was raised, and the Arab merchant passed under the knee, riding on a camel with spear erect, and did not touch it. I cut off one corner of the purple-blue shawl of one of them; and we could not move away. He said unto me: '[If] you have, peradventure, taken something from them, return it; for we have a tradition that he who takes anything from them cannot move away.' I went and returned it; and then we were able to move away. When I came before the Rabbis they said unto me: Every Abba is an ass and every Bar Bar Hana is a fool. For what purpose did you do that? Was it in order to ascertain whether [the Law] is in accordance with the [decision of] Beth Shammai or Beth Hillel? You should have counted the threads and counted the joints.

(Soncino translation)

The Talmud apparently assumes that whichever of the opinions is correct is how tzitzit were worn all the way back to the time of Moses in the wilderness.