We used to perform the Maghrib prayer with the Prophet (may Allah's peace and blessings be upon him) and when one of us left he could still see where his arrow landed

This Hadīth clarifies that the Prophet (may Allah's peace and blessings be upon him) would often perform the Maghrib prayer at the beginning of its time period and clarifies that this is a part of the Sunnah. Evidence for this is that they would finish the prayer while there was still light enough for them to see where the arrows that they had shot landed.