Musannaf Ibn Abi Shayba 3:1718

Jarir narrated to us, from Mansur, from Ibrahim, who said: They used to dislike that a man should wait when the mu'adhdhin says "the prayer has been established" (qad qamat al-salah) and the Imam is not with them. And they used to dislike waiting for the Imam while standing. And it used to be said that this is *al-sumud* (standing idly).