Shaykh Muḥammad ibn Ṣāliḥ al-Uthaymīn:

I will also point out for you an important issue. When you take an oath, say after it – In shā Allah [if Allah ﷻ wills], even if your friend doesn’t hear you because if you do say In shā Allah, Allah will ease the affair for you to fulfil your oath, and if He [Allāh ﷻ] wills that [your oath] doesn’t take place, then there is no expiation on you.

And this is a great benefit! So if you – for example say to someone, ‘By Allah you shall not slaughter for me, and then you say silently [In shā Allah], thereafter he slaughters for you, then there’s no blame on you, nor is there an expiation of an oath on you. Likewise the opposite, if you say, ‘By Allah ﷻ I shall slaughter…’ then you say silently [In shā Allah] and the listener doesn’t hear you, then if you don’t slaughter there is no expiation on you. Due to the statement of the Prophet  ﷺ “Whoever swears about an oath and says: ‘If Allah wills [In shā Allah], then there is no breaking of the oath against him.” (refer to al-Tirmidhī], this is a great benefit, so always keep it [In shā Allah] on your tongue in order that you gain two benefits:

First benefit: So that your affairs are be made easy for you.

Second benefit: That if you break your oath, then there is no expiation on you.