Musannaf Ibn Abi Shayba 3:5796

Hushaym narrated to us, he said: Harun ibn Zadhi (Zadhawayh) informed us, from Mujahid, who said: A man came to Ibn Abbas and said: "I and a companion of mine were on a journey, and I used to perform the prayer in full (itmam), while my companion used to shorten it (qasr)." Ibn Abbas said to him: "Rather, you were the one who was shortening, and your companion was the one who was performing it in full."