Wa alaykum alsalam wa rahmatullah,
Al-Balathuri wrote a couple of books on history. One is called Ansab Al-Ashraf (a book about lineages and biographies) and the other is Futooh Al-Buldan (a book about conquests). He did include narrations from the Prophet - peace be upon him - in his books. He is not seen as a credible compiler, for nobody from the scholars have strengthened him nor weakened him, which makes his status: Unknown (majhool).
Edit: Also, Ibn Hajar included him in Lisan Al-Mizan, which suggests that he is weak.