It is narrated from Sayyiduna Ibn Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) would sometimes give some of the detachments more, compared to the whole army, and the khumus (one-fifth) was obligatory on all the property.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12806