Assalamualaikum and hello.
The sources of law during the period were three namely, the Holy Qur'an, the Sunnah, and the ijtihad. The Holy Qur'an is the first primary source of law manifesting the words of Allah the Almighty. It is actually a source of guidance for the Muslims. Law was revealed either during the Mecca or Medina periods. An example of law revealed during the Mecca period was prohibition on murdering children. An example of law revealed during the Medina period was fosterage. The Sunnah on the other hand is the words, actions, and tacits approvals of the Prophet Muhammad (pbuh). One of its functions in law was to specify legal details of any law revealed by the Holy Qur'an. For instance, the Holy Qur'an reveals that prayers are an obligation. Thus, the Sunnah specifies on how to perform prayers. An example of law derived from the Sunnah during the preiod was custody and visitation rights after divorce. The ijtihad was normally performed by the Prophet (pbuh) when there were legal situations in which there were not yet revelations from Allah the Almighty. Normally after performing the ijtihad, revelations came down either to approve or correct the ijtihad. Thus, the Prophet (pbuh) also approved or disapproved some ijtihad of the Sahabah or his Companions. Nevertheless, any ijtihad performed by the Sahabah are considered as part of the Sunnah. In short, the sources were actually considered as an important foundation for future developments of law.
Wassalam and thanks.