Whenever the Messenger of Allah ﷺ went out on a military campaign, he had a select share of the spoils which he could take from wherever he wished. Safiyyah was from that share. When he did not personally take part in the campaign, a share would still be allotted to him, but he was not given the right to choose.
حدثنا محمود بن خالد السلمي حدثنا عمر يعني ابن عبد الواحد عن سعيد يعني ابن بشير
كان رسول الله ﷺ إذا غزا كان له سهم صاف يأخذه من حيث شاءه فكانت صفية من ذلك السهم وكان إذا لم يغز بنفسه ضرب له بسهمه ولم يخير
No references are available.
Graded, this isnad is daif. Bayhaqi reported it in 6/304 from the hadith of Abu Dawud with this chain. The chain is mursal and daif.