Musannaf Ibn Abi Shayba 3:3587

Yazid ibn Harun narrated to us, he said: Al-Asbagh ibn Zayd informed us, he said: Thawr ibn Yazid narrated to us, from the one who informed him, that Abu Dharr said: "Pray four [rak'ahs] after the Dhuhr [prayer], for if you forget the Asr [prayer], it will be in its place."