Riyad-us-Saliheen 504
'Aishah (May Allah be pleased with her) reported: When Messenger of Allah (PBUH) died, his armour was mortgaged with a Jew for thirty Sa' (measures) of barley.
[Al-Bukhari and Muslim]
In the Hadith, there is a mention of the asceticism of the Prophet (PBUH) and also a permissibility for transactions with the people of the Book (the Jews and the Christians). That is why he mortgaged his coat of mail with a Jew and took a loan against it.