Short answer
No, saying wallahi (by Allah) is not haram, because it is a valid oath by Allah. But using it all the time, as a habit or a filler, is disliked, since the Quran tells you to guard your oaths. Swearing by Allah to a lie is a major sin.
Swearing by Allah when there is a reason is permitted. Swearing often without need is makruh, because it cheapens Allah's name and leads to careless or false oaths. A deliberate oath about the future that you break needs kaffarah.
The four schools agree on this.
Explanation
Wallahi means 'by Allah'. It is an oath, and the Prophet ﷺ taught that if you must swear, swear by Allah or stay silent. So the word itself is not haram. In fact it is the correct way to swear, unlike swearing on your mother or your life, which is not allowed. See what wallahi means.
The problem is how people use it. Many say wallahi in every sentence, to win an argument, to sound serious, or as a habit. The Quran says 'guard your oaths' and tells us not to make Allah's name an excuse in our oaths. The Prophet ﷺ warned traders against swearing a lot when selling: it may move the goods, but it wipes out the blessing. The scholars took from this that swearing often without need is makruh (disliked).
What makes it serious:
- Swearing to something false: saying wallahi about a past event you know is untrue is a major sin. See swearing by Allah and lying.
- Breaking a sworn promise: 'Wallahi I'll come tomorrow', then not coming, needs kaffarah. See kaffarah for breaking an oath.
- Careless habit (laghw): 'No, wallahi, no' said out of habit about something past or present that you believe is true is the kind of oath Allah does not hold you to. But in the Hanafi school a habitual 'wallahi I will...' about the future is still a binding oath, even if said lightly, and breaking it needs kaffarah.
Wallahi billahi tallahi is the same oath said three ways, for emphasis. Hanafi books count oath words repeated without a joining 'and' as one oath; if you need to know how many kaffarahs you owe, ask a scholar. Train yourself to say 'yes' and 'no' plainly, and keep Allah's name for when it truly matters. The four schools agree on this ruling.
The evidence
Quran, Surah Al-Maaida 5:89
لَا يُؤَاخِذُكُمُ ٱللَّهُ بِٱللَّغْوِ فِىٓ أَيْمَٰنِكُمْ وَلَٰكِن يُؤَاخِذُكُم بِمَا عَقَّدتُّمُ ٱلْأَيْمَٰنَ ۖ فَكَفَّٰرَتُهُۥٓ إِطْعَامُ عَشَرَةِ مَسَٰكِينَ مِنْ أَوْسَطِ مَا تُطْعِمُونَ أَهْلِيكُمْ أَوْ كِسْوَتُهُمْ أَوْ تَحْرِيرُ رَقَبَةٍۢ ۖ فَمَن لَّمْ يَجِدْ فَصِيَامُ ثَلَٰثَةِ أَيَّامٍۢ ۚ ذَٰلِكَ كَفَّٰرَةُ أَيْمَٰنِكُمْ إِذَا حَلَفْتُمْ ۚ وَٱحْفَظُوٓا۟ أَيْمَٰنَكُمْ ۚ كَذَٰلِكَ يُبَيِّنُ ٱللَّهُ لَكُمْ ءَايَٰتِهِۦ لَعَلَّكُمْ تَشْكُرُونَ ﴿٨٩﴾
Allah will not impose blame upon you for what is meaningless in your oaths, but He will impose blame upon you for [breaking] what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days [is required]. That is the expiation for oaths when you have sworn. But guard your oaths. Thus does Allah make clear to you His verses that you may be grateful.
Allah tells believers to guard their oaths, which the scholars read as not swearing often.
Quran, Surah Al-Baqara 2:224
وَلَا تَجْعَلُوا۟ ٱللَّهَ عُرْضَةًۭ لِّأَيْمَٰنِكُمْ أَن تَبَرُّوا۟ وَتَتَّقُوا۟ وَتُصْلِحُوا۟ بَيْنَ ٱلنَّاسِ ۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌۭ ﴿٢٢٤﴾
And do not make [your oath by] Allah an excuse against being righteous and fearing Allah and making peace among people. And Allah is Hearing and Knowing.
Allah's name should not be used lightly in oaths.
Sahih al-Bukhari 2679
Sahih... من كان حالفا فليحلف بالله أو ليصمت ...
"Whoever must swear, let him swear by Allah or keep silent."
Oaths by Allah are the valid kind, and silence is the alternative.
Sahih Muslim 1607
Sahih... إياكم وكثرة الحلف في البيع فإنه ينفق ثم يمحق ...
"Beware of swearing a lot when selling, for it helps the sale and then wipes out the blessing."
Frequent swearing is blameworthy even when it seems to help.
Where the ruling is recorded
- al-Hidaya, Kitab al-Ayman. Divides oaths into laghw, ghamus and binding oaths, with their rulings.
- Radd al-Muhtar (Ibn Abidin), Kitab al-Ayman. Swearing often without need is disliked.
Checked by our team. For a more accurate answer to your own situation, consult a scholar. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.