Don't listen to Christian preachers. The word "Tawhid" has nothing to do with the Christian trinity doctrine. Rather the opposite in fact. The trinity is not even mentioned in the Bible because it is a Christian tradition that originates from European (Greek and Roman) paganism, later adopted by Christians.
Tawhid in Arabic means attributing oneness to Allah (swt) and describing Allah (swt) as being one and unique, with no partner or peer in his essence and attributes.
Tawhid means monotheism that is believing in one true God who has no associates or parters. It's root word is waahid, ahad and waheed.
This word (Tawheed) and its derivatives are used with this meaning in the Qur’aan and Sunnah. For example:
“Say (O Muhammad): He is Allaah, (the) One [Ahad].Allaah‑us‑Samad [Allaah — the Self‑Sufficient Master, Whom all creatures need, (He neither eats nor drinks)].He begets not, nor was He begotten.And there is none co‑equal or comparable unto Him” (al-Ikhlaas 112:1-3)
“And your Ilaah (God) is One Ilaah [ilaahun waahidun] (God — Allaah), Laa Ilaaha illa Huwa (there is none who has the right to be worshipped but He), the Most Gracious, the Most Merciful” (al-Baqarah 2:163)
“Surely, disbelievers are those who said: ‘Allaah is the third of the three (in a Trinity).’ But there is no Ilaah (god) (none who has the right to be worshipped) but One Ilaah [ilaahun waahidun] (God —Allaah). And if they cease not from what they say, verily, a painful torment will befall on the disbelievers among them” (al-Maa'idah 5:73)
The above verses from the Quran we can read that the concept of Oneness is found in the Quran. And the word Tawheed has derived from It's root word Waahid, Ahad and Waheed.
Did Prophet Muhammed Pbuh and His Companions know about the word Tawheed and the Answer is Yes!. We have Sahih Hadiths where the word Taweed is Explicity Mentioned, which tells us The Prophet Muhammed Pbuh and his Companions were well aware of the Word Taweed and this Word was used during the beginning of Islam. Let's cite a few hadiths to show Tawheed was a well known Word.
It was narrated from ‘Aishah and Abu Hurairah that when the Messenger of Allah (ﷺ) wanted to offer a sacrifice, he brought two large, fat, horned, black-and-white, castrated rams. He slaughtered one on behalf of his nation, for whoever testified to Allah with monotheism (بِالتَّوْحِيدِ )and that he had conveyed (the Message), and he slaughtered the other on behalf of Muhammad and the family of Muhammad (ﷺ). ( Sunan Ibn Majah Vol 4 Book 26 Hadith 3122)
The Word Tawheed is used in Arabic بِالتَّوْحِيدِ
“The Messenger of Allah came with two sheep both with big horns, then he reclined to one side and said: Bismillah wa Allahu Akbar, O Allah! Be with Muhammad and his family, then reclined to the other and said: Bismillah wa Allahu Akbar, O Allah! Be with Muhammad and his Ummah who testify/bear witness to you of/with TAWHEED and testify/bear witness to this statement/declaration. (Narrated by Jabir ibn Abdullah, collected by Ibn Hajr al-Asqalani, Matalib al-Aliya volume 3, page 32 (Hassan))
The Word Tawheed is used in Arabic ، بالتوحيد
“Amr bin Shu`aib narrated from his father, who narrated from his grandfather that al-Aas bin Wa’il took an oath during the pre-Islamic times (Jahiliyyah) that he would slaughter one hundred goats and that Hisham bin al-`Aas slaughtered his share of 50 goats. Then `Amr bin al-`Aas asked the Prophet (P) about that, and he said, “As for your father, if he affirmed TAWHIDand you fast and give charity on his behalf, it would benefit him.”
أما أبوك فلو كان أقر بالتوحيد، فصمت وتصدقت عنه نفعه ذلك “. (Imam Ahmad narrated this, See silsilah saheeha (484))
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