Because I focus on Norse stuff, I know of Ibn Fadlan as a famous primary source. However, I don't know anything about him besides that he was a soldier on some kind of diplomatic expedition who encountered a group of Norsemen who traveled down the Volga and recorded his meetings with them, most famously an account of a funeral.
I can answer this!
Ibn Fadlan was the secretary to a higher ranking Abbasid emissary. He was part of a mission to the king of the Volga Bulgars, who had requested Abbasid aid against the Khazars. This probably had something to do with the conflicts between the Rus and Khazar peoples that were going on at this time.
It's worth mentioning that the Abbasids and the Khazars had been in conflict before so the Muslims responding to this request isn't out of the ordinary. The Khazars had been responsible for numerous raids into Abbasid territory and two previous conflicts with the Umayyad caliphate
At some point in their journey, the Abbasid envoy strayed off the path they had been following and encountered several ethnic groups on their way to the Bulgar's capital. This is what is covered in Ibn Fadlan's writings.
Ibn Fadlan himself was supposedly an expert on Islamic law and served as the envoy's adviser on matters of the Shariah. According to Hermes (2012); the Bulgars would receive financial and martial support in exchange for their religious and political tribute to the Caliphate.
Sources and recommended reading:
The Vikings, by Johannes Brondsted
Ibn Fadlan: An Arab Among the Vikings of Russia
Ibn Fadlan and the Russiyyah, by James E. Montgomery
The Other in Medieval Arabic Literature and Culture, by Nizar F. Hermes
As a follow up question, what really happened to the money he was supposed to deliver to the king of the Volga Bulgars?