The Conditions for an Act of Worship to be Accepted by, Taken from the lecture Lamhah 'an al-Firaq adh-Dhaallah by Shaykh Saalih al-Fawzaan

...The qualification for every deed must fulfill two conditions in order to be accepted by Allah and in order for the person who does (the deed) to be rewarded:

The first condition is that the deed is done sincerely for Allah the Mighty and Majestic alone.

The second is that it must be done in accordance to the statements and actions of the Messenger, peace be upon him. The Most High says: {Nay, whoever submits his whole self to Allah and is a doer of good he will get his reward with his Lord; on such shall be no fear, nor shall they grieve.} al-Baqarah: 112

The meaning of submitting one’s face is doing every action sincerely for Allah. And the meaning of doing every action perfectly means in accordance with the statements and actions of the Messenger, peace be upon him. Allah the Glorious and Lofty commands us to be united upon the Book and the Sunnah (the ways of the Prophet Muhammad, peace be upon him), and He prohibits us from division and disputes.

Likewise the Prophet, peace be upon him, commanded us to unite upon the Book and the Sunnah (the ways of the Prophet Muhammad, peace be upon him), and he prohibited us from division and disputes since unity based on the Book and the Sunnah (the ways of the Prophet Muhammad, peace be upon him) contains benefit that can occur immediately and at other times can occur later. (And he prohibited us from division and disputes) since division contains harm that can occur immediately and at other times can occur later, whether in this life or in the hereafter.

Therefore the issue is in dire need of strict concern because as time goes on more groups appear, more indoctrination appears, more deviant sects and false ideas come about, and various parties increase. The obligatory matter for the Muslim to do is look, whatever is in conformity with the Book of Allah and the Sunnah (the ways of the Prophet Muhammad, peace be upon him) then he takes it from the one who comes with that which is (based on the Book and the Sunnah), whoever he may be because following the truth is the real objective of the believer.

As for the one who opposes what the Messenger, peace be upon him, was upon, then he leaves that person, even if he is from his own party or with one whom he ascribes himself with (from his group). The reason is because he opposes the Book and the Sunnah (the ways of the Prophet Muhammad, peace be upon him). A person wants to be successful (in the hereafter), he doesn’t want destruction for himself.

Politeness has no place here, the issue is an issue concerning paradise or hell. A person does not promote flattery and politeness or blind partisanship or his desires to side alongside those who oppose the group that is upon the way of the prophet, peace be upon him, and the general body of Muslims (Ahlus-Sunnah wal-Jamaa’ah). In doing so, he only harms himself. He expels himself from the way of success and follows the path to destruction.

The group that is upon the way of the prophet, peace be upon him, and the general body of Muslims (Ahlus-Sunnah wal-Jamaa’ah) are not harmed by those who oppose them, whether you are with them or oppose them. If you are with them then all praise is for Allah, and they are happy with this, because they only want good for the people. If you oppose them, you do not harm them. Based on this the Prophet, peace be upon him, said:

(There will always be a group from my nation who will be upon the truth. They are not harmed by the one who humiliates them until the issue of Allah comes (the Day of Judgment) and they are still upon the truth.) Related by Muslim. 

Therefore the one who opposes the group that is upon the way of the prophet, peace be upon him, and the general body of Muslims (Ahlus-Sunnah wal-Jamaa’ah) only harms himself.

Taken from the lecture Lamhah 'an al-Firaq adh-Dhaallah 
(A Look into the Deviant Sects) 
Shaykh Saalih al-Fawzaan
Translated by Thomas Maldonado