Musannaf Ibn Abi Shayba 3:5388

Affan narrated to us, he said: Abu 'Awanah narrated to us, from 'Umar ibn Abi Layla, from his father, he said: 'Uthman ibn 'Affan used to lengthen the first two rak'ahs of the Dhuhr prayer, reciting in them [the length of] Surah al-Baqarah.