Qatadah reports the following:

I asked Anas b. Malik, "Who collected the Quran during the lifetime of the Prophet?" He replied, "Four persons, all of them from theAn$ar (Helpers): Ubayy b. Kab, Muadh b. Jabal, Zayd b. Thabit, and Abu Zayd."٩

According to Masri١q, one day Abd Allah b. Umar remembered Ad
Masud, and said, "I continue to love him. I heard the Prophet say, Learn the Quran from four: AbdAllah b. MasUd, Salim, Muadh, and Ubayy b. Kab." ١٠
Al-NasaI reports a tradition with a sound chain of transmission going back to Abd Allah b. Umar, who said, "I collected the Quran and I used to read [all of] it every night. The Prophet came to know about it and told me: Read it in a month...."١١
We shall presently cite the tradition from Ibn Sad regarding the collection of the
Quran by Umm Waraqa.
It is possible to argue that the notion of collection (jam) in the aforementioned traditions implies collection in the memory and not in recording (tadwfn) the revela‌ tion in a volume. However, such an assertion rests on no evidence. Additionally, during the Prophets time, more people than could be counted memorized the text of the Quran. How then could they be limited to four or six persons, as these traditions do? Anyone who has examined the history of the Companions and the Prophet would know with certainty that the Quran existed in the form of a complete collection during the Prophets time, and that the number of people engaged in collecting it was fairly siz‌ able. However, as for the tradition related by al-Bukhan on the authority of Anas, which says that the Prophet died and no one had collected the Quran except four-Abi١
Darda, Muadh b. Jabal, Zayd b. Thabit, and AbU Zayd-it has to be rejected and discarded because it contradicts all the preceding traditions, including those reported by al-Bukhan himself. Moreover, this tradition is hard to believe, for how could Anas, the narrator of the report, have information about every individual Muslim at the time of the Prophets death-they were large in number and spread out in the region-to enable him to limit the persons who collected the Quran to four? This assertion is nothing more than a conjecture about the unknown and an opinion without knowledge.
To recapitulate, in view of the abovementioned traditions regarding the Qurans existence in the collected form during the Prophets time, how can one believe that AbU Bakr was the first to collect it after assuming the caliphate? If we do accept the validity of such a claim, then the question arises as to why he ordered Zayd and Umar to collect it from palm branches, flat stones, and the memories of men? Why did he not take it from AbdAllah, Muadh, and Ubayy, who were alive at the time of the collection, and who, along with Salim, were, according to the Prophets instructions, the persons from whom the Quran should be acquired? It is true that Salim had been killed in the battle of Yamama, and thus the Quran could not be acquired from him. Nevertheless, Zayd, as it appears from this tradition, himself was one of the compil‌ ers of the Quran. As such, there was no need to look for, or ask, someone else, espe‌ cially as he was a wise and trusted man, as AbU Bakr himself said. Besides all these points, the tradition about the "two things of high estimation" (thaqalayn) indicates that the Quran existed as a complete collection during the Prophets time, as we shall explain further below.