Hadhrat Anas(r.a) says that on one occasion the Aws and Khazraj tribes boasted to each other. The Aws said, "Amongst us was the person whom theangels bathed (after his martyrdom). He was
Handhala bin Raahib (r.a). Alsoamongst us was the person for whom the throne of Allaah shook (when he passed away). He was Sa'd bin Mu'aadh(r.a). Amongst us was also the person
who was protected by a swarm of wasps (when the Mushrikeen intended to mutilate his body). He was Aasim bin Thaabit bin Abil Aflah(r.a). We also had in our ranks the person whose testimony was allowed (by Rasulullaah(saw)) in place of the testimony of two people. He was Khuzayma bin Thaabit
(r.a). May Allaah be pleased with all of them."
In response to this, the members of the Khazraj said, 'We have four persons who memorised the entire Qur'aan during the lifetime of Rasulullaah(r.a).
There were none besides them who achieved this honour. They were Zaid bin
Thaabit(r.a), Ubay bin Ka'b (r.a), Mu'aadh bin Jabal (r.a) and Abu Zaid (r.a). May Allaah be pleased with all of them."
{Source: Abu Ya'la,Bazzaar, Tabraani all narrating from reliable sources as confirmed by Haythami (Vol.10 Pg.41). Abu Awaana and Ibn Asaakir have also reported the narration, as quoted in Muntakhab(Vo1.5 Pg. 139}.