Allāh subhānahu wa ta`āla said:

"So whoever hopes for the Meeting with his Lord, let him work righteousness and associate none as a partner in the worship of his Lord." (al-Kahf 18:110)

"And who can be better in religion than one who submits his face (himself) to Allāh (i.e. follows Allāh's Religion of Islāmic Monotheism); and he is a Muhsin (a good-doer). And follows the religion of Ibrāheem (Abraham) Haneefā (Islāmic Monotheism - to worship none but Allāh Alone)." (an-Nisā’ 4:125)

And (hadeeth) in Al-Bukhāri and Muslim: from the narration of `Ā’ishah who narrated from the Prophet (sallāhu `alayhi wasallam), that he said: "Whoever introduces into this affair of ours that which is not from it will have it rejected". And in the narration from Muslim: "Whoever does a deed that is not ordered by us will have it rejected".

So every action that is not in agreement with the Sunnah does not increase its doer except in distance away from Allāh, for verily Allāh ta`ālā is only worshipped with what He has ordered to be worshipped with, not with opinions and desires.

From Tafseer Ibn Katheer: Allāh's Love is Attained by Following the Messenger

Soorah Āli Imrān 3:31-32: "Say (O Muhammad to mankind): "If you (really) love Allāh, then follow me (i.e. Muhammad), Allāh will love you and forgive you your sins. And Allāh is Oft-Forgiving, Most Merciful.''

Say: "Obey Allāh and the Messenger.'' But if they turn away, then Allāh does not like the disbelievers."

This honourable āyah judges against those who claim to love Allāh, yet do not follow the way of Muhammad . Such people are not true in their claim until they follow the Sharee`ah (Law) brought by Muhammad and his religion in all his statements, actions and conditions. This is why Allāh said here: "Say (O Muhammad to mankind): "If you (really) love Allāh, then follow me, Allāh will love you...'" meaning, what you will earn is much more than what you sought in loving Him, for Allāh will love you. Al-Hasan Al-Basri and several scholars among the Salaf commented, "Some people claimed that they love Allāh. So Allāh tested them with this Āyah”

Allāh then said: "And forgive you your sins. And Allāh is Oft-Forgiving, Most Merciful." meaning, by your following the Messenger, you will earn all this with the blessing of his mission. Allāh next commands everyone: "Obey Allāh and the Messenger.'' But if they turn away" by defying the Prophet, "then Allāh does not like the disbelievers." Thus, testifying that defiance of the Messenger's way constitutes kufr. Indeed, Allāh does not like whoever does this, even if he claims that he loves Allāh and seeks a means of approach to Him, unless, and until, he follows the unlettered Prophet, the final Messenger from Allāh to the two creations: mankind and the Jinn. This is the Prophet who, if the previous Prophets and mighty Messengers were to have been alive during his time, they would have no choice but to follow, obey him, and to abide by his Law.

[For further details, see: al-Qawl-ul-Mufeed Fee Adillatit-Tawheed - Shaykh Muhammad al-Wasabi

and Ref: Tafseer Ibn Katheer, Volume 2]

Six Conditions of Ibādah

An act of worship cannot be considered a righteous action, except if two conditions are fulfilled with regards to it, the first is al-ikhlās (sincerity) and the second is al-mutāba’ah (following the Prophet sallAllāhu `alayhi wasallam). And al-mutāba’ah cannot be fulfilled except when the action is in agreement with the Sharee`ah in six affairs [Majmoo’ul-Fatāwā (5/253-254) of Ibnul-’Uthaymeen]:

1: as-Sabab (reason, motive)

When the person worships Allāh with act of worship that is accompanied by a motive that is not legislated by the Sharee`ah, then it is an innovation that is rejected back to its doer. An example of that is when some of the people commemorate the twenty-seventh night of Rajab under the pretence that it was the night in which the Messenger of Allāh (sallAllāhu `alayhi wa sallam) ascended. So tahajjud (the night Prayer) is worship. However, when it is accompanied with this motive, it becomes an innovation. This is because this action of worship is built upon a motive that has not been confirmed in the Sharee`ah. And this description – the conformity of the worship to the Sharee`ah in motive – is an important affair that clarifies many innovations from amongst those that are thought to be from the Sunnah, yet they are not from the Sunnah.

2. al-Jins (type)

It is inevitable that the act of worship be in conformity to the Sharee`ah in its jins (type). So if the person worships with an act of worship whose type is not legislated, then it is not accepted. An example of that is if a man were to slaughter a horse, then this sacrifice would not be correct, because it has opposed the Sharee`ah in the type. So the blood sacrifices cannot be done, except with cattle, grazing livestock, camels, cows and sheep.

3. al-Qadr (quantity)

If the person increases in the Prayer that is obligatory, then we say that this is an innovation that is not accepted, because it opposes the Sharee`ah in quantity. With all the more reason, if the person prays the noon Prayer for example as five units, then his Prayer is not correct by agreement.

4. al-Kayfiyyah (manner, mode)

If a man performs ablution, and he begins with washing the feet, then he wipes his head, then he washes his hands, then his face, then we say that his ablution is null and void, because it opposes the Sharee`ah in manner.

5. az-Zamān (time)

If the person slaughters in the first days of Dhul-Hijjah, then his blood sacrifice will not be accepted due it opposing the Sharee`ah in time. And I have heard that some of the people slaughter sheep during the month of Ramadān in order to draw closer to Allāh with blood sacrifice. So this action in this manner is an innovation, because there is nothing with regards to drawing closer to Allāh with slaughtering, except for blood sacrifice, gifts and the ’aqeeqah. As for slaughtering in Ramadān along with the belief that one will attain the reward of slaughtering, such as the blood sacrifice in ’Eedul-Adhā, or slaughtering for the sake of meat, then this is permissible.

6. al-Makān (place)

If a man performs i’tikāf in other than a mosque, then his i’tikāf is not correct. And that is because the i’tikāf cannot occur, except in the mosques. And if a woman says, ‘I wish to perform i’tikāf in the musallā (place of Prayer) in the house,’ then her i’tikāf is not correct due to opposition of the Sharee`ah in place. And from the examples of that is if a man desires to perform tawāf. So he finds that the area of tawāf has become too confined and he finds that whatever is around it has become too confined. So he starts to perform tawāf from behind the mosque, then his tawāf will not be correct, because the place of the tawāf is the house.

[Source : Six Conditions of Ibādah, by Shaykh Muhammad bin Sālih al-`Uthaymeen]