Imam al-Shafi'i (may Allah have mercy on him) said: "I do not know of any opponent who disputes that the Messenger of Allah (peace and blessings be upon him) ruled the diyah (blood money) to be upon the 'aqilah (the killer's paternal kinsmen, the 'asabah); and this (is so well known among the general public that it) is more than a hadith narrated by particular individuals, and we have also mentioned it from the hadith narrated by particular individuals."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / Q16367
Uthman bin Muhammad bin Uthman bin al-Akhnas bin Shariq said: I took this document from the family of Sayyiduna Umar (may Allah be pleased with him); it was attached to the letter on sadaqah which he (may Allah be pleased with him) had written to the officials: "In the name of Allah, the Most Gracious, the Most Merciful. This is a document from Muhammad the Prophet (peace and blessings be upon him) between the Muslims and the believers of Quraysh and Yathrib, and those who follow them, join them and strive (in jihad) with them: that they are one ummah, distinct from (other) people. The Muhajirun of Quraysh, according to their existing custom, pay blood money among themselves, and they ransom their captives with kindness and justice among the believers. And Banu Awf, according to their existing custom, pay their blood money as before, and every group ransoms its captive with kindness and justice among the believers." Then, in this same order, he mentioned Banu al-Harith, then Banu Sa'idah, then Banu Jusham, then Banu al-Najjar, then Banu Amr bin Awf, then Banu al-Nabit, then Banu al-Aws. Then he said: "And the believers shall not leave anyone among them who is weighed down by debt without giving him (help), with kindness, in ransom or blood money."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16369
It is narrated from Sayyiduna Amr bin Awf (may Allah be pleased with him) that in the letter of the Prophet (peace and blessings be upon him) this was also written: "Every group shall ransom its captive with kindness and justice among the believers; and it is upon the believers that they do not leave anyone among them who is weighed down by debt without giving him (help) in ransom or blood money."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الديات / 16370