Complex sentence: Shurhabil's corps, that were stationed at Yamamah was to reinforce Khalid's corps, in addition to this Abu Bakr assembled a fresh army of Ansar and Muhajireen in Medina that joined Khalid's corps at Butah.
Split into simpler sentences: Shurhabil's corps, that was stationed at Yamamah, was to reinforce Khalid's corps. In addition to this Abu Bakr assembled a fresh army of Ansar and Muhajireen in Medina that joined Khalid's corps at Butah.