I said to Abu Umar, "What were they writing?"He replied, "The sermon they heard from him that day."
حدثنا علي بن سهل الرملي قال
قلت لأبي عمرو ما يكتبوه قال الخطبة التي سمعها يومئذ منه
No references are available.
[Its chain of transmission is sahih] see the previous hadith * Abu Amr is Uzai.