Musannaf Ibn Abi Shayba 35:55

Ghundar narrated to us, from Shu'bah, from al-Hakam, from Ibrahim, from 'Alqamah, from 'Abdullah, from the Prophet (peace be upon him), that he prayed Dhuhr as five [rak'ahs]. It was said to him, "You have prayed five." So he performed two prostrations after he had finished the prayer (taslim). And he mentioned that Abu Hanifah said: "If he did not sit during the fourth [rak'ah], he must repeat the prayer."