It was compiled by Imam Abu Dawood al-Tayalisi (Sulaymān ibn Dāwūd, 133 H/750-1 CE?
It is one of the oldest Musnad ( a Hadith book with full isnāds, also organized by Companion) written.
It is written in the second century of the Islamic Calendar and written before the most authentic book of Hadiths (narrations of Muhammad) that are Sahihain (Sahih al-Bukhari & Sahih Muslim).
The Musnad (مسند) are collections of Hadiths which are classified by narrators, and therefore by Sahabas (companions of Muhammad).
It seems that the collection is not directly the work of the Imam, but what his pupil Yunus ibn Habib assembled from what the Sheikh transmitted to him.