Shaykh Muhāmmād Ibn Abdul Wahaab may Allāh have mercy on him said:
Worship is not considered as worship except it’s accompanied with Tawheed [ Oneness of Allāh in His worship ] , As Prayer is not considered as prayer except is accompanied with purity [ Ablution ] , If Shirk [ Associating partners with Allāh in His worship ] enter in a worship it corrupts and nullifies it .
Ad-Durar al-Sanniyya [2/23]
Translation: Knowledge and Action