Al-Khatib al-Baghdadi reported: Abu Hanifa, may Allah have mercy on him, said to his student Abu Yusuf one day, “Woe to you, O Ya’qub! Do not write down everything you hear from me. Indeed, I may have an opinion today and abandon it tomorrow, or I may have an opinion tomorrow and abandon it the day after tomorrow.”
Source: Tārīkh Baghdād 15/544
عن الخطيب البغدادي قال يوما أَبُو حنيفة رحمه الله لأبي يوسف ويحك يا يَعْقُوب لا تكتب كل ما تسمع مني فإني قد أرى الرأي اليوم فأتركه غدا وأرى الرأي غدا وأتركه بعد غد
15/544 تاريخ بغداد للخطيب البغدادي