Ubay ibn Ka'b
Ubayy was born in Medina (then known as Yathrib), into the tribe of the Banu Khazraj. He was one of the first to accept Islam and pledge allegiance to Muhammad at Aqabah before the migration to Medina, becoming one of the Ansar. He participated in the battle of Badr and other following engagements.
He acted as a scribe of Muhammad, writing letters for him. Ubayy was one of the few who put the Qur'anic suras into writing and had a Mushaf of his own. Following Muhammad's death, he was one of the twenty five people who knew the Qur'an completely by heart.
He was part of the consultative group (mushawarah) to which the caliph Abu Bakr referred many problems. It included Umar, Uthman, Ali, Abd-al-Rahman ibn Awf, Muadh ibn Jabal, Ubayy ibn Kab and Zayd ibn Thabit.
Umar later consulted the same group when he was caliph. Specifically for fatwas (legal judgments) he referred to Ali ibn Abi Talib, Uthman, Ubayy and Zayd ibn Thabit.
Ubayy died in the year 649 CE (30 AH during the caliphate of Uthman.)
Sunnis include him in the Hadith of Learning Qur'an from four people, in which according to the hadith, Muhammad said: "Learn the Qur'an from four persons: Abd-Allah ibn Mas'ud, Salim Mawla Abu-Hudhayfah, Ubay ibn Kab and Muadh ibn Jabal."
He is well regarded by Shi'as.