Given the task definition and input, reply with output. Given a question and a context passage, generate the answer having the word or phrase from the context passage. Here, the answer should be a shortest continous span from the passage.

Context: Umm Kulthum bint Abi Bakr (Arabic: ) (born c.634 (13 AH)), was a daughter of Abu Bakr and Habiba bint Kharija.
Question: Who fathered Umm Kulthum bint Abi Bakr?
abu bakr