The wealth of Banu Nadir was among what Allah granted to His Messenger as fay, since the Muslims had not ridden out against it on horses or camels. It belonged exclusively to the Messenger of Allah ﷺ. He would spend from it on his family's yearly maintenance, then place what remained into weapons and mounts as provision for the cause of Allah.
كانت أموال بني النضير مما أفاء الله على رسوله مما لم يوجف المسلمون عليه بخيل ولا ركاب، فكانت الرسول الله ﷺ خاصة ينفق على أهله منها نفقه سنته، ثم يجعل ما بقي في السلاح والكراع، عدة في سبيل الله.
Bukhari related it in the tafsir (4885) and Muslim in jihad and expeditions (1757), both of them through Sufyan, from Amr, from Zuhri, from Malik bin Aws bin Hadathan, from Umar, who said, and he mentioned it. The wording given is Bukhari's, and Muslim's wording is similar.