Musannaf Ibn Abi Shayba 3:1700

Abu Usama narrated to us, from Musa ibn Ubaydah, from Sa’d ibn Ibrahim, who said: Perform the prayer of the *Asal* (late afternoon) when the shadow returns, at the time of the call for the Dhuhr prayer. Whoever performs it, it is as if he had performed the night vigil (*tahajjud*) during the night.