Shahabad district or Arrah district, headquartered at Arrah (now part of Bhojpur District, Bihar) was a Bhojpuri speaking district in western Bihar, India, making the western border of Bihar with Uttar Pradesh. In 1972, the district was split into two districts: Bhojpur and Rohtas. Kaimur District was carved out from Rohtas in 1991 and Buxar District from Bhojpur in 1992. Historically, the geographic area was the eastern part of the Mahajanapada Kingdom of Kashi, with the Son River separating it from Magadh. The districts in erstwhile Shahabad are as follows: