Urwah bin al-Zubayr relates that Zayd bin Thabit (may Allah be pleased with him) said to Marwan bin al-Hakam: Do you recite "Qul huwa Allahu ahad" and "Inna a'taynaka al-kawthar"? He said: Yes. Zayd bin Thabit said: "I swear by it! I saw the Messenger of Allah (peace and blessings be upon him) reciting the longer of the two long surahs."
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4796