Shaykh al-Islām Ibn Taymiyyah (d. 728 AH) said: “There is a party among the scholars of Kūfah who hold the opinion that vocalising the intention is mustaḥabb (recommended) for a person who is performing wuḍū’, ghusl, [about to] pray, or carry out any other act of worship. They claim that vocalising intentions for these acts of worship is stronger than merely an intention an individual holds in his mind. However, there are no scholars who have ruled the vocalisation of intentions to be wājib. Rather, the scholars of Madīnah have not [even] given it a ruling of mustaḥabb. Their [opinion] is correct [in this matter].
The companions of [Imām] Aḥmad have two opinions, one of which is that vocalisation is a bidʿah, as the Messenger of Allāh (ﷺ) and his Companions never vocalised their intentions. Rather, they would start their ṣalāh with the takbīr without saying anything before it. Even when teaching the Ṣaḥābah to pray, [the Prophet (ﷺ)] only taught them to open their ṣalāh with the takbīr. So, this is indeed a bidʿah in the sharīʿah. It is also an error with regards to intention as an [individual] must naturally house an intention within himself before he carries out any action. Consequently, vocalisation of that intention is completely futile. It is like the vocalisation of the intention of a person who is about to eat, drink, marry, travel or other than that [in that it has no bearing on the action itself].”
Source: Majmūʿ al-Fatāwā 10:360-361
Translated by: Riyāḍ al-Kanadī