Reference: Ibn ‘Abdul Barr in Al-Intiqaa’ fi Fadaa’il ath-Thalaathah al-A’immah al-Fuqahaa’ (p. 145), Ibn al-Qayyim in I’laam al-Muwaqqi’een (2/309), Ibn ‘Aabideen in his Footnoes on Al-Bahr ar-Raa’iq (6/293) and in Rasm al-Mufti (pp. 29, 32) & Sha’raani in Al-Meezaan (1/55) with the second narration. Similar narrations exist on the authority of Abu Haneefah’s companions Zafar, Abu Yoosuf and ‘Aafiyah ibn Yazeed; cf. Eeqaaz (p. 52). Ibn al-Qayyim firmly certified its authenticity on the authority of Abu Yoosuf in I’laam al-Muwaqqi’een (2/344).
Imaam Abu Haneefah (rahima-hullaah) said:
It is haraam for someone who does not know my evidence to give fataawa on the basis of my words.
Another narration adds:
… for we are mortals: we say one thing one day, and take it back the next day.