Making an oath, a vow or a covenant in one’s heart
18-4-2011 | IslamWeb
Question:
-if i made a vow/promise/oath/covenant to Allah that i would wear hijab but i do it in my head because i didn’t know that it wont be valid if its not done verbally, i thought it will be valid if you do it in your head. Now i know that it wont be valid if its not done verbally so must i still keep it? I know i will be sinful for not wearing hijab but in this case will i be held accountable for breaking my vow/oath/covenant/promise? i know if something is already obligatory or prohibited, then it remains an obligation or prohibition, but my question is if i did the vow/promise/oath/covenant to Allah that i would wear hijab but i do it in my head because i didn’t know that it wont be valid if its not done verbally, now everytime i dont wear my hijab i will be sinful for not wering it but also for breaking my promise? -what is the fatwa in this case: if for example i made a promise/vow/oath/covenant with god that i will stop gossiping about people but i cant remember if i did it verbally or in my head? In this case i know i will be sinful for gossiping but also for breaking my vow/oath/covenant/promise?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
First of all, Hijab is an Islamic obligation on a Muslim woman and she should take the initiative to abide by this obligation in any case, and if she does not do so, she is sinful. There are texts from the Quran and the Sunnah that prove that Hijab is an obligation, and you may refer to them in Fatwa 84053 and 84341.
However, uttering the oath or vow or covenant (that you wear Hijab) strengthens its obligation even more. But, these matters do not take effect unless one utters them. The evidence about this is that Abu Hurayrah 
narrated that the Prophet said: "Allaah has forgiven my nation for the whispers that they have except if they speak about it or act according to it.” [Al-Bukhari and Muslim]. For more benefit, refer to Fatwa 104740.
What we mentioned about the Hijab also applies to backbiting and the like, because backbiting is forbidden in principle by the Book of Allaah and the Sunnah of the Prophet and whoever commits backbiting is sinful. For more benefit on the evidence of the prohibition of backbiting, please refer to Fatwa 81748.
Therefore, if a Muslim makes a vow, an oath, or a covenant to abandon backbiting, then the prohibition is more confirmed in his regard and the sin is graver if he breaches his oath. Again, these matters do not take effect except by uttering them as we already mentioned.
Finally, if a Muslim doubts whether or not he uttered a vow, an oath, or a covenant, then in principle he did not utter it because the rule of the scholars is: non-existence is a fundamental presumption, which is a branch of the great Fiqh rule: “Certainty is not invalidated by doubt.”
Allaah Knows best.