It is narrated from Qatadah (may Allah have mercy on him) that when the Messenger of Allah (peace and blessings be upon him) himself took part in a battle, he had a chosen share which he would take from wherever he wished; Safiyyah (may Allah be pleased with her) was from that share. And when he did not go out to battle himself, a share was allotted to him, but he did not choose.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قسم الفيء والغنيمة / 12753