Ibn Isḥāq's Kitāb al-maghāzī, is a compilation of reports on events that occurred during or prior to the life of the Prophet Muhammad, including his military expeditions. Ibn Isḥāq “arranged [the reports] into what he deemed to be their correct chronological order and to which he added his own comments” adding a chain of narration to most reports. This excerpt is on the birth of the Prophet.
For more on the text, see Nicolai Sinai and William Montgomery’s entry on the Prophet Muhammad in Encyclopedia Britannica.