All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
No doubt that continuing to say Taqabala Allah (May Allah accept your act) intending a form of worship or a way to get closer to Allah after each prayer to some Muslim brothers is an innovation as this has not been reported from the Prophet or his companions.
Nobody should worship Allah with anything unless there is sound evidence that it has been established by the Quran or the Sunnah as part of Islam.
‘Aa’ishah narrated that the Prophet said: “Whoever introduces into this matter of ours (Islam) that which is not part of it, it will be rejected.” [Al-Bukhari and Muslim]
But if the person says this expression from time to time not intending to get closer to Allah, then there is no harm in that.
Also, saying this expression on ‘Eed day is lawful. Jubayr Ibn Mutim said: “The companions of the Prophet used to say to one another on the ‘Eed day: ‘Taqabala Allahu Minna wa Minkum’ (May Allah accept our good deeds and yours).” Ibn Hajar said: “This Hadeeth has a good chain of narrators.”
For more benefit, please refer to Fatwa 88493.
Allah knows best.