let us see a narration from musnad ahmad

Abu huraira said that holy prophet asws asked sahaba: do you know what gheebat/back biting is? They said: allah and his prophet asws knows better. He said: gheebat is when you speak of your brother with a defect which is not there in him. Someone asked: what if my brother has that defect and I speak of it in his absence, what is the order then? He said: if he has that defect, then it is gheebat, otherwise it is bohtan/false charge .

[musnad ahmad, urdu version, vol 4,page 9-10]

Molvi zafar says sahih as per muslim and ibn haban

Sheikh shoaib says isnaad sahih on condition of muslim [12/57/7146]