Assalaamu alaykum. I will describe two situations, but both of them are related to the same topic and ask similar questions. 1) "I will never leave praying Tahajjud (night prayer)," or, " I promise you O, Allaah, that I will always pray Tahajjud," or, " By Allaah, I will always pray Tahajjud," or the like. I am not sure nor do I remember clearly whether I ever took the above mentioned oath or made such a promise. It is possible that it was only a firm intention in my heart or words to remain committed to praying Tahajjud but not actually a promise or oath. Sometimes, I feel that I actually took an oath and sometimes its the opposite.I do offer tahajjud regularly but there are times when I overslept or travelled so now I am worry.
2) My aunt took an oath that if Allaah blesses her with a child, she will pray two voluntary units of prayer with every prayer her whole life. Now the child is almost 18 years old, and she missed her voluntary prayers for an unknown number of times that amounts to a lot. I have the following questions:
1. What to do if a person is uncertain of his oath as to whether it was taken or not?
2. Suppose that I actually took an oath, then:
a) Does the expiation have to be offered each time I broke my oath by not praying Tahajjud or once only?
b) If a person has broken such an oath, is he still obliged to continue fulfilling that oath even after it has been broken or does the obligation cease to exist as soon as it is broken?
3. Can the obligation to offer an expiation be waved if it involves a terrible amount of hardship if one is required to offer an expiation for each time. Please specifically mention the ruling regarding a sentence that is neither a vow or oath nor a promise, but just: "O Allaah, I will always pray Tahajjud," and which does not include the expressions, 'I promise', 'By Allaah', or the like. May Allaah reward you.

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

A vow is not valid and effective unless it is made with words that imply commitment. As for saying 'I will do such-and-such,' then this is not a binding vow. Having doubts whether a vow was made or not entails that the person is not obliged to do anything in this regard. Thus, you come to know that you are not obliged to do anything for not performing Tahajjud (voluntary night prayer) because you cannot decisively say that you said words that imply a binding vow.

If we assumed that a vow or oath was actually made, then you would be obliged to offer an expiation as soon as the vow or oath is broken. The oath would thus be dissolved, and you would not be obliged to pay another expiation afterward unless the words that you used indicated repetition. For instance, if you had said, 'Every time I neglect such-and-such, I will offer an expiation or charity.' In such case, you would be obliged to offer an expiation or charity every time you neglect that thing.

For more benefit, please refer to fataawa 119549and 138168.

Allaah knows best.