Musannaf Ibn Abi Shayba 38:82

Ishaq ibn Mansur narrated to us, from Muhammad ibn Rashid, from Ja'far ibn 'Amr ibn Umayyah, from Muslim ibn al-Ajda' al-Laythi—and he was among those who witnessed Siffin—he said: 'Ammar would go out between the two ranks, and the banners had already been brought out. He would call out until they could hear him with his loudest voice: "Go forth to Paradise! The wide-eyed houris have adorned themselves."