The properties of Banu Nadir were among what Allah granted to His Messenger ﷺ as spoils that the Muslims had not gained by riding horses or camels into battle. They belonged exclusively to the Messenger of Allah ﷺ. He would spend from them on his family's annual maintenance, then place whatever remained toward weapons and mounts as provision for the cause of Allah.
كانت أموال بني النضير مما أفاء الله على رسوله ﷺ مما لم يوجف المسلمون عليه بخيل ولا ركاب، فكانت لرسول الله ﷺ خاصة ينفق على أهله منها نفقه سنته، ثم يجعل ما بقي في السلاح والكراع عدة في سبيل الله.
Bukhari reported it in the tafsir (4885) and Muslim in the jihad (1757), both from the hadith of Sufyan, from Amr, from Zuhri, from Malik bin Aws bin Hadathan, from Umar bin Khattab, who said, and then he mentioned it.