He traced it back to the Prophet ﷺ, who said, "A wasq is sixty saa."
حدثنا عبد الله بن سعيد الكندي حدثنا محمد بن عبيد الطنافسي عن إدريس الأودي عن عمرو بن مرة عن أبي البختري
رفعه إلى النبي ﷺ قال الوسق ستون صاعا
No references are available.
[Its chain is daif due to a break in transmission]Reported by Abu Dawud, Zakat, chapter: What zakat is obligatory on, no. 1559. He said, "Abu Bakhtari did not hear from Abu Saeed," and Ibn Khuzayma doubted its soundness. The hadith has an addition found in Abu Dawud and others, which is sahih. See Sunan Nasai, and Bayhaqi, no. 2485.