Imaam Muhammad Ibn Saalih al-'Uthaymeen (d.1421H) - rahimahullaah - said, “Sincerity (ikhlaas) towards Allaah means that the person desires by his worship closeness to Allaah the Exalted and reaching His Abode of Nobility (i.e. Paradise), such that the servant must be sincere towards Allaah the Exalted in his intended purpose. He must be sincere towards Allaah the Exalted in his love. He must be sincere towards Allaah in his exaltation. He must be sincere to Allaah the Exalted outwardly and inwardly. He does not seek anything with his worship, except for the Face of Allaah the Exalted and reaching His Abode of Nobility.” 
Stated al-’Allaamah Muhammad Ibn Saalih al-’Uthaymeen (d.1421H) – rahimahullaah, “And in order that it will be known – O brothers – that al-mutaaba’ah cannot be actualized, except when the action is in agreement with the Sharee’ah in six affairs:
The First: as-Sabab (reason, motive); So when the person worships Allaah 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 Allaah (sallallaahu ’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.
The Second: al-Jins (type); so 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.
The Third: al-Qadr (quantity); so 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.
The Fourth: al-Kayfiyyah (manner, mode); so 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.
The Fifth: az-Zamaan (time); so 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 Ramadaan in order to draw closer to Allaah with blood sacrifice. So this action in this manner is an innovation, because there is nothing with regards to drawing closer to Allaah with slaughtering, except for blood sacrifice, gifts and the ’aqeeqah. As for slaughtering in Ramadaan along with the belief that one will attain the reward of slaughtering, such as the blood sacrifice in ’Eedul-Adhaa, or slaughtering for the sake of meat, then this is permissible.
The Sixth: al-Makaan (place); so if a man performs i’tikaaf in other than a mosque, then his i’tikaaf is not correct. And that is because the i’tikaaf cannot occur, except in the mosques. And if a woman says, ‘I wish to perform i’tikaaf in the musallaa (place of Prayer) in the house,’ then her i’tikaaf 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 tawaaf. So he finds that the area of tawaaf has become too confined and he finds that whatever is around it has become too confined. So he starts to perform tawaaf from behind the mosque, then his tawaaf will not be correct, because the place of the tawaaf is the house.
So the act of worship cannot be considered a righteous action, except if two conditions are fulfilled with regards to it, the first is al-ikhlaas (sincerity) and the second is al-mutaaba’ah (following the Prophet). And al-mutaaba’ah cannot be fulfilled, except with the six affairs that have been previously mentioned.” 
: Refer to Majmoo’ul-Fataawaa (7/112) of Ibnul-’Uthaymeen.
: Refer to Majmoo’ul-Fataawaa (5/253-254) of Ibnul-’Uthaymeen.