Musannaf Ibn Abi Shayba 3:5384

Ma'n ibn 'Isa narrated to us, from Khalid ibn Abi Bakr, who said: I prayed behind al-Qasim, and he used to lengthen the first two [rak'ahs] more than the last two: the first two of the Dhuhr, the first two of the 'Asr, the first two of the Maghrib, and the first two of the 'Isha'.