Musannaf Ibn Abi Shayba 3:5893

Waki’ narrated to us, he said: Ibn ‘Awn narrated to us, from Raja’ ibn Haywah al-Kindi, he said: Thabit ibn al-Simt—or al-Simt ibn Thabit—was on a journey in a state of fear, and the time for prayer arrived, so they prayed while mounted. Al-Ashtar descended, so he said: "What is the matter with him?" They said: "He descended and prayed." He said: "What is the matter with him? He opposed [the Sunnah], may he be opposed."