Imaam Abu Haneefah Rahimahullah said:

"When a hadeeth is found to be saheeh, then that is my madhhab."

Ibn 'Aabideen in al-Haashiyah (1/63) and in his essay Rasm al-Mufti (1/4 from the Compilation of the Essays of Ibn 'Aabideen), Shaikh Saalih al-Fulaani in Eeqaaz al-Himam (p. 62) and others. Ibn 'Aabideen quoted from Sharh al-Hidaayah by Ibn al-Shahnah al-Kabeer, the teacher of Ibn al-Humaam.