Task: Question Answering
Input (English): Passage: Based on earlier transmitted reports, in the year 632, after the demise of Muhammad a number of his companions who knew the Quran by heart were killed in a battle by Musaylimah, the first caliph Abu Bakr (d. 634) decided to collect the book in one volume so that it could be preserved. Zayd ibn Thabit (d. 655) was the person to collect the Quran since "he used to write the Divine Inspiration for Allah's Apostle". Thus, a group of scribes, most importantly Zayd, collected the verses and produced a hand-written manuscript of the complete book. The manuscript according to Zayd remained with Abu Bakr until he died. Zayd's reaction to the task and the difficulties in collecting the Quranic material from parchments, palm-leaf stalks, thin stones and from men who knew it by heart is recorded in earlier narratives. After Abu Bakr, Hafsa bint Umar, Muhammad's widow, was entrusted with the manuscript. In about 650, the third Caliph Uthman ibn Affan (d. 656) began noticing slight differences in pronunciation of the Quran as Islam expanded beyond the Arabian Peninsula into Persia, the Levant, and North Africa. In order to preserve the sanctity of the text, he ordered a committee headed by Zayd to use Abu Bakr's copy and prepare a standard copy of the Quran. Thus, within 20 years of Muhammad's death, the Quran was committed to written form. That text became the model from which copies were made and promulgated throughout the urban centers of the Muslim world, and other versions are believed to have been destroyed. The present form of the Quran text is accepted by Muslim scholars to be the original version compiled by Abu Bakr. Question: Which caliph decided to preserve the Quran as a single book after some of Muhammad's companions were killed in battle?
Output (English): Abu Bakr