Musnad Ahmad 26:880

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from al-Hakam, from Ibrahim, from 'Alqamah, from 'Abdullah, from the Prophet, may Allah bless him and grant him peace, that he prayed Dhuhr as five [rak'ahs]. It was said to him, "Has the prayer been increased?" The Prophet, may Allah bless him and grant him peace, said, "And what is that?" They said, "You prayed five." So he performed two prostrations after he had said the taslim. Shu'bah said: And I heard Sulayman and Hammad narrating that Ibrahim did not know whether he had prayed three or five.