Musnad Ahmad 1007:132

Waki’ narrated to us, Ali—meaning Ibn al-Mubarak—narrated to us, from Yahya ibn Abi Kathir, from Abdullah ibn Abi Qatadah, from his father, who said: The Messenger of Allah (ﷺ) used to recite in the first two rak'ahs of the Dhuhr prayer, making us hear the verse sometimes, and he would prolong the first rak'ah and shorten the second. And he would recite in the first two rak'ahs of the Asr prayer. And he would prolong the first rak'ah of the Fajr prayer and shorten the second.