Qatadah said “When the Apostle of Allaah ﷺ participated in battle there was for him a special portion which he took from where he desired. Safiyyah was from that portion. But when he did not participate himself in his battle, a portion was taken out for him, but he had no choice.” (Using translation from Abū Dāʾūd 2993)
كَانَ رَسُولُ اللهِ ﷺ إِذَا غَزَا كَانَ لَهُ سَهْمٌ صَافٍ يَأْخُذُهُ مِنْ حَيْثُ شَاءَ فَكَانَتْ صَفِيَّةُ مِنْ ذَلِكَ السَّهْمِ وَكَانَ إِذَا لَمْ يَغْزُ بِنَفْسِهِ ضُرِبَ لَهُ بِسَهْمِهِ وَلَمْ يَخْتَرْ