[The Ruling on One Who Makes Something Obligatory upon Himself by Intention Alone]

Answer – and Allah is the One who grants success: It is not binding to fulfill what is mentioned in the question. That is because supererogatory acts of worship do not become obligatory merely by the intention of a vow; rather, they only become obligatory if he makes them binding upon himself by spoken wording, such as saying, “I have vowed to Allah the Exalted to fast two days,” or the like. It has been mentioned in the commentary of al-Qāḍī Zayd that there is consensus on this.
It is possible to adduce as evidence for this the ḥadīth: “Indeed, Allah has overlooked for my community what they speak to themselves about, so long as they do not speak it aloud or act upon it.”
For further benefit, we say: The actions of the heart are: knowledge and assumption (ẓann) and what follows from them; faith and disbelief and what follows from them; will, resolve, and intention.
Inner self-talk (ḥadīth al-nafs) is something else.
And the action of the tongue is informing about what is in the heart, and performative speech (inshāʾ), and legal “effects” (īqāʿāt).
Among the performative utterances are the contracts of marriage and sale and all other exchanges. Among the “effects” are divorce, manumission, vows, endowments, oaths, and the like.
The Wise Lawgiver has attached the rulings of those performative utterances and those “effects” to the verbal expression of the tongue and its articulation of them.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2