Sheikh Yûsuf b. `Ali al-Tarîf
This phrase: "Taqabbal Allah," means "May Allah accept it." It is a supplication that Allah accepts our worship from us. It is not established by the Sunnah. If a Muslim says this to another without considering it a ritual practice and without doing so all of the time, then there is nothing wrong with it. It simply falls under the category of the general supplications that we make in our own words.
What is forbidden is to make it a constant practice, as if these words were ritually prescribed for us. It is wrong to make anything an act of ritual worship except for what is specifically prescribed for us by the Qur'an and Sunnah.
The Prophet (peace be upon him) said: "Whoever does something that is not established by our command, then it is rejected." [Sahîh al-Bukhârî (2697) and Sahîh Muslim (1718)]
The Prophet (peace be upon him) also said: "Follow my Sunnah and that of the Rightly-Guided Caliphs who come after me. Adhere to it fast (as if you were) biting down on it with your molars. Beware of newly-introduced things. Everything that is newly introduced is an innovation, and every innovation is misguidance." [Sunan Abî Dâwûd (4607), Sunan al-Tirmidhî (2676), and Sunan Ibn Mâjah (42)]
Supplications are allowed in our own words. We can call upon Allah and beseech Him with our needs in any words that express our meaning. It only becomes an innovation if we turn our words into a sort of formula that we commit to memory and repeat on various occasions. Such ritual supplications must be established by the sacred texts.
Therefore, if someone feels the need after every congregational prayer to say "Taqabbal Allah" to the next person, as if doing so were part of the prayer, then this would be wrong.
Otherwise, it is simply a custom, like a customary greeting offered after prayer, and not an act of formal worship.
And Allah knows best.
Source: Islam Today