Answered by Shaykh Faraz Rabbani
If I mentioned a negative thing about another person without mentioning them thinking that the listener didn’t know them. It turns out that the listener did know who I was talking about. Is this backbiting (gheebat)?
It would depend on whether it could be reasonably expected whether or not the listener could be reasonably expected to know the person being talked about, given what you said. So check what you said. It it was something from which, in retrospect, you could expect the listener to figure out the person mentioned, then consider it backbiting. Repent, and (ideally), pray 2 rakats of the repentance prayer. [Which entails doing wudu, praying 2 rakats, and seeking Allah’s forgiveness after it. Whoever performs it has been promised forgiveness, in a rigorously authenticated hadith related by Imam Ahmad in his Musnad, Tirmidhi, and others.] Otherwise, it is still an error in judgement and outwardly a sin, even if it is not actually one. Even in such cases it is taqwa to feel remorse, and seek forgiveness, and learn one’s lesson, as the scholars mention.
Imam al-Barkawi said in his masterpiece on the operationalization of taqwa, al-Tariqa al-Muhammadiyya,
“Backbiting is to mention the negative points of a specific brother who is known to the the one being addressed, or to indicate them or make them known by the hand or any other limbs.”
Allama Abu Sa`id al-Khadimi explained,
“(A specific brother who is known to the the one being addressed) for the who is not specified or known would not be backbiting.” [al-Bariqa al-Mahmudiyya fi Sharh al-Tariqa al-Muhammadiyya, 3: 183)
Safety lies is holding fast to the counsel of the Prophet (Allah bless him and give him peace), “Whoever believes in Allah and the Last Day should say that which is good or remain silent.”
And he said, “Whoever is silent is saved.”
And, “From the excellence of a man’s Islam is to leave that which does not concern him.” The scholars explain “that which concerns him” as being anything in which there is a real worldly or next-worldly benefit.
It is also reported by Anas (may Allah be pleased with him) that the Messenger of Allah (Allah bless him and give him peace) said, “Silence is wisdom, though few are silent.” (Bayhaqi with a weak chain, and it has been authenticated that it is from the sayings of Luqman the Wise.)
Imam Zafar Usmani explained: “Silence is wisdom, though few are silent,” is a confirmation of the practices of the Sufis, for limiting speech is among the spiritual struggles (mujahada) that they built their way upon. There are several Prophetic hadiths in praise of silence in place of unnecessary speech. Among them is the hadith of Ibn `Umar that the Prophet (Allah bless him and give him peace) said, “Whoever is silent is saved.” (Tirmidhi, who said gharib, and Tabarani, with a reliable chain of narrators). And Muhammad ibn al-Hasan reported at the end of his Athar, from Abu Hanifa, from Hammad, that Ibrahim [al-Nakh`i] said, “Tribulations are caused by speech.” Qadi Ibn Bahlul said, “Do not utter words you like not for it may be / that the tongue utter words and they come to be.” Imam Malik reports from Aslam that our master `Umar (may Allah be pleased with him) came to our master Abu Bakr (may Allah be pleased with him) one day while the latter was holding his tongue. `Umar said to him, “May Allah forgive you (for what you are doing)!” Abu Bakr replied, “Verily this has cast me into (countless) problems.” And Ahmad, Nasa’i, Ibn Majah and Tirmidhi elate with a chain that Tirmidhi deemed good and rigorously authentic (hasan sahih), from Mu`adh ibn Jabal that he asked, “O Messenger of Allah, are we taken to task for what we say?” He (Allah bless him and give him peace) said, “May your mother be bereaved of you, O Mu`adh! Are people dragged into the Fire on their faces – or their noses – for anything but what their tongues have reaped?” Therefore, it is upon the intelligent person to be aware of his times, directed to what is of consequence, and careful about his tongue. And whoever considers his speech to be of his actions talks little except that which concerns him. [Book of Sufism & Good Character, being the final chapter of I`la’ al-Sunan, an unpublished translation.]
And Allah knows best.