Hadith: The property of Banu An-Nadīr was among what Allah, the Almighty, bestowed upon His Messenger (may Allah's peace and blessings be upon him) as spoils and for which the Muslims did not have to fight, neither on horses nor on camels.

عن عمر بن الخطاب رضي الله عنه قال: «كانت أموال بَنِي النَّضِيرِ: مِمَّا أَفَاءَ الله على رسوله صلى الله عليه وسلم مِمَّا لم يُوجِفْ الْمسلمون عليه بِخَيْلٍ وَلا رِكَابٍ وكانت لرسول الله خالصاً، فكان رسول الله صلى الله عليه وسلم يَعْزِلُ نفقة أَهْلِهِ سَنَةً، ثُمَّ يجعل مَا بقي في الْكُرَاعِ، وَالسلاحِ عُدَّةً فِي سبيل الله عز وجل ».
[صحيح] - [متفق عليه]

‘Umar ibn Al-Khattāb (may Allah be pleased with him) reported: The property of Banu An-Nadīr was among what Allah, the Almighty, bestowed upon His Messenger (may Allah's peace and blessings be upon him) as spoils for which the Muslims did not have to fight, neither on horses nor on camels. It belonged exclusively to the Messenger of Allah (may Allah's peace and blessings be upon him). So, he put aside his family’s expenses for one year from it and whatever left was spent on horses and weapons (for fighting) in the cause of Allah.
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

When the Prophet (may Allah's peace and blessings be upon him) emigrated to Madīnah, he found groups of Jews around it. So, he made a treaty with them and called for a truce, so they would continue to follow their religion and not wage war against him or assist his enemies against him. A man from the Companions, ‘Amr ibn Umayyah Ad-Damri (may Allah be pleased with him), killed two men from Banu ‘Āmir, thinking that they were enemies of the Muslims. So, the Prophet (may Allah's peace and blessings be upon him) assumed the burden of paying the blood money for the two men. He set off for the village of Banu An-Nadīr seeking their assistance to settle the blood money. While he was sitting in one of their markets waiting for their assistance, they broke their treaty and decided to kill him. Revelation of their betrayal came to him from the heavens, so he left the village, pretending that he needed to relieve himself, and set off towards Madīnah. When he was late in coming back, his Companions set out in his footsteps and he informed them of the betrayal of the Jews (may Allah disgrace them). He besieged them in their village for six days, until they reached an agreement that they would go to the Levant, Hīrah, and Khaybar. So, their property was an easy booty, gained without the Muslims facing any hardship, as they did not have to fight with horses or camels. Their property was for Allah and His Messenger. The Prophet (may Allah's peace and blessings be upon him) put aside the expenses of his family for one year and spent the rest on the general welfare of the Muslims. That was the spendig priority at that time: the preparation of horses and weapons for fighting in the cause of Allah. Every period of time has its own priority of spending, considering the public interest.