بسم الله الرحمن الرحيم
O' reader please know -may Allah have mercy on you- that the tawheed in which Allah has made binding upon His servants before the obligation of prayer and fasting is the tawheed with respect to your worship. So do not supplicate to anyone except Him alone and without partner. Do not supplicate to the prophet -sallahu alayhi wa salaam- or anyone besides him. As Allah says:
و أن المساجد لله فلا تدعوا مع الله أحداً
And the places of worship are for Allah alone so do not supplicate to anyone along with Allah. [Al-Jinn: 18]
and He says
قل إنما أنا بشر مثلكم يوحى إلى أنما إلاهكم إلاهٌ واحدٌ فمن كان يرجوا لقاء ربه فليعمل عملاً صالحاًو لا يشرك بعبادة ربه أحداً
Say (O' Muhammad): I am a man like you all, it has been revealed to me that you deity is only one. So whoever hopes in the meeting with his lord let him do righteous deeds and not associate a partner in worship with his lord. [Al-Kahf: 110]
O' reader please know that the idol worshipers to whom which Allah's messenger -sallahu alayhi wa salaam- fought, the description of their idolatry was that they supplicated to Allah and along with Him to their idols (as well), also to the righteous like Esa, his mother, and the Angels. They'd say, "these are our intercessors with Allah!" Although they (knew and ) acknowledged the fact that Allah is the one who brings benefit and causes harm, He's the one that regulates the affairs of His creation just as Allah mentions in His statement:
قل من يرزقكم ن السماء و الأرض أمّن يملك السمع الأبصار و من يُخرج الحىّ من الميّت و يُخرج الميّت من الحىِّ و من يدبّر الأرض فسيقولون الله فقل أفلا تتقون
Say (O' Muhammad): Who is it that provides for you from the sky and the earth? Or who is it that owns hearing and sight? And who brings forth the living from the dead and the dead from the living? And who regulates the affairs? They will say, "Allah." Will you not have fear? [Yunus: 31]
So if you are aware of this O' reader, and you know that their supplication to the righteous and their reliance upon them wherein they would say (as an excuse), "we don't desire (by way of our supplication to other than Allah) nothing more than intercession" than (you should know) that indeed our Prophet -sallahu alayhi wa salaam- fought them in order for worship to be solely for Allah alone and that the deen all of it is for Him.
In addition you're aware that this is tawheed, that which is more binding upon you than prayer and fasting. Allah will forgive whoever comes with it on the day of resurrection and will not forgive whoever is ignorant of it despite him being a devout worshiper.
You are aware that the associating of partners with Allah is that which He will not forgive to whoever does so, and with Allah it is greater than adultery and killing even though the person who does such only desires being brought closer to Him.
So along with this you realize another affair, it being that the majority of people are unaware of these (previously mentioned) things, especially those who've been dubbed to be people of knowledge. So if it is said, "we single out Allah alone for worship, no one brings benefit or causes harm except Him. The righteous do not do so." You'll realize (after making such a statement) that they know nothing as it relates to tawheed except the tawheed of the disbelievers. That being tawheed of Lordship( and with that knowledge) you'll recognize Allah's tremendous favor upon you only if you actualizes it. Thus whomsoever turns to Allah however he doesn't know tawheed or he knows it but doesn't act in accordance with it, then such a person will abide in the fire forever even if he be from the most devout in worship. Allah says
إنه من يشرك بالله فقد حرم الله عليه الجنة و مأواه النار و ما للظالمين من أنصار
Indeed whosoever associates a partner in worship with Allah, then Allah has forbidden Paradise to him, and his abode is the fire, and there are no helpers for the oppressive. [Al-Maidah: 72]
And Allah knows best. May Allah's peace and praise be upon Muhammad, his family, and companions.
Source: توحيد العبادة
Translated by Najeeb ibn Yusuf Al Anjelesi