What are the evidences in Maliki jurisprudence for the existence of two Adhans for the Fajr prayer, and when is the second Adhan?

It is permissible to give the Adhan for Fajr prayer before its time, and scholars have adduced as proof what has been authentically reported from the Prophet, peace and blessings be upon him, that Bilal used to give the Adhan at night, and by his saying, peace and blessings be upon him: "Indeed, Bilal gives the Adhan at night, so eat and drink until Ibn Umm Maktum gives the Adhan." Jurists have differed in determining the earliest time for the Adhan for Fajr before dawn. Some have held that it is permissible from the middle of the night, while others have held that it is permissible from the last sixth of the night.