The Salaf on speaking negatively about a Muslim
Sufyan bin Hussain said: I mentioned a (muslim) man negatively in the presence of Iyas bin Muawiyah(Taabi'i, Judge of Basra),so he looked me in the face & said:"Did you join the fight against the Romans?"I said: No,then he said:
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"the Sindhis, Indians & Turks (mongols)?" I said no.
He replied: "the Sindhis, Indians & Turks are safe from you but your Muslim brother is not safe from you?!" After that moment, I never did that again.
{Al-Bidayah Wan-Nihayah by Ibn Kathir (13/121)