What are the foundational sources for Islamic law?

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The foundation of jurisprudence rests upon the Book (Quran), the Sunnah (Prophetic tradition), and the consensus (Ijma). Evidence from the Quran is established by the verse concerning thieves: "As for the male thief and the female thief, amputate their hands" (Quran 5:38). Evidence from the Sunnah includes the narration from Aisha that the Messenger of Allah, peace and blessings be upon him, stated, "The hand is amputated for [the theft of] a quarter Dinar and above." Furthermore, the Prophet, peace and blessings be upon him, stated, "The people before you were only ruined because when a noble among them committed theft, they would let him go, and when a weak person committed theft among them, they would cut off his hand" (agreed upon sources). Muslims are in consensus regarding the obligation of cutting off the hand of a thief in principle.