The Prophetic Basis
The foundational case (agreed upon in the sources): a man from Khaybar was found murdered in disputed territory between the Jewish community of Khaybar and the Muslim emigrants. No witnesses. The Prophet asked the claimants (the Ansar, who were his kin): “Will you swear 50 oaths and thereby establish your right to blood money for him?” They refused (saying they had not witnessed it). He then asked the Khaybar community to swear 50 oaths of innocence, which they did, and blood money was paid by the Prophet’s treasury.
This case established the qasama procedure in the Sunna.
The Schools
Malikis and Hanbalis: the victim’s relatives (awliya’ al-dam) swear 50 oaths that the accused killed their relative, based on strong circumstantial evidence (lawth). This can lead to both blood money and (in the Maliki school) retaliation.
Hanafis: the oaths are sworn by the accused to establish their innocence — it is an exculpatory oath, not an accusatory one. Only blood money, not retaliation, can result.
Shafi’is: the qasama leads to blood money only, never retaliation — the principle being that the certainty required for qisas can never be achieved through oath-based evidence.
The Tension
The qasama represents Islamic law’s attempt to handle a practically common situation (killed bodies, no eyewitnesses) without either ignoring the death entirely or executing someone without proper evidence. The result is a compromise: financial compensation (diya) can be established through the collective oath even when retaliation (qisas) cannot be. The Shafi’i and Hanafi refusal to allow qisas based on qasama reflects the principle: punishments requiring certainty demand evidence at the highest standard.
See also: Fiqh Al Jihad Ethics, Ilm Al Usul, Fiqh Al Wasatiyyah, Ilm Al Qiyas, Fiqh Al Ijma