Imam al-Shafi'i (may Allah have mercy on him) said: "Ghanimah (spoils of war) and fay' are alike in that, from the whole of both of them, the khumus (one-fifth) belongs to those whom Allah Almighty has named in the two verses."
And he said in his old (opinion): "The khumus is taken only from that wealth which was obtained by spurring on (horses and camels) against it (that is, by fighting)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / Q12720
Abu Jamrah says: I heard Ibn Abbas (may Allah be pleased with them both) say that when the delegation of Abd al-Qays came to the Messenger of Allah (peace and blessings be upon him), he (peace and blessings be upon him) asked: "Who are the people?" They said: "From Rabi'ah." He (peace and blessings be upon him) said: "Welcome to the people, without any regret." They said: "O Messenger of Allah (peace and blessings be upon him)! We are a tribe of Rabi'ah, we have come to you after great hardship, and between us and you stands the tribe of Mudar of the disbelievers, so we can reach you only in the sacred months. So give us a clear command, to which we may also invite those we have left behind, and by which we may enter Paradise." The Messenger of Allah (peace and blessings be upon him) said: "I command you to do four things and forbid you four things. I command you to believe in Allah alone. Do you know what belief in Allah is? It is to bear witness that there is no god but Allah and that Muhammad (peace and blessings be upon him) is the Messenger of Allah, to establish the prayer, to pay the zakah, to fast in Ramadan, and that you give the khumus (one-fifth) from the spoils of war. And I forbid you four things: al-dubba' (gourds), al-hantam (green glazed jars), al-naqir (hollowed-out wooden vessels) and al-muzaffat (pitch-coated vessels)" and sometimes he said the word al-muqayyar, and he said: "Remember them and invite those you have left behind to them as well."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12720
Mu'awiyah bin Qurrah narrates from his father (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) sent his father, that is, Mu'awiyah's grandfather, to a man who had married his father's wife; he struck off his neck and took the khumus (one-fifth) of his wealth.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12722
Umar bin Abd al-Aziz (may Allah have mercy on him) wrote that whoever asks where the wealth of fay' should be spent, the answer is: as Umar bin al-Khattab (may Allah be pleased with him) ruled, which the Muslims then considered just and in agreement with the saying of the Messenger of Allah (peace and blessings be upon him): "Allah Almighty has placed the truth upon the tongue and heart of Umar." Umar (may Allah be pleased with him) fixed the stipends and granted protection (dhimmah) to the people of all religions in return for the jizyah, and he neither levied the khumus on it nor treated it like the spoils of war.
The narration of Umar bin Abd al-Aziz from Umar bin al-Khattab (may Allah be pleased with him) is munqati' (broken), and Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12723