In the book titled "Contemporary Zakat" by Mahmoud Abu-Saud, he quotes a Hadith which I'm rephrasing here: "The Prophet Muhammad (SAW) order a Zakat Collector by the name of Abu Hathama to take whatever is beyond the living cost."
From this Abu Saud validates the idea that the Nisab corresponds to the average family yearly living cost.
Where can I find this Hadith? I need the reference. If someone has access to his book, please verify if this Hadith is authentic.