Musannaf Ibn Abi Shayba 1:1355

Jarir narrated to us, from Abd al-Aziz ibn Rufay’, from ‘Ata’, from Ibn Abbas, who said: You delay the Dhuhr and hasten the ‘Asr, and you perform ghusl once. And you delay the Maghrib and hasten the ‘Isha’, and you perform ghusl once. Then you perform ghusl for the Fajr, then you combine between them.