Tawheed: The core of Ibadah.

 Tawheed: The core of Ibadah. 

Tawheed is the core message of all prophets. 

Allah says, 

An-Nahl 16:36

And We certainly sent into every nation a messenger, [saying], "Worship Allāh and avoid ṭāghūt."[1] And among them were those whom Allāh guided, and among them were those upon whom error was [deservedly] decreed. So proceed [i.e., travel] through the earth and observe how was the end of the deniers.

Taghut here means polytheism or shirk. 

Tawheed is to call Allah in help, assistance, guidance, help, sacrifice for Allah alone. 

Allah says, 

Al-An'am 6:162

قُلْ إِنَّ صَلَاتِى وَنُسُكِى وَمَحْيَاىَ وَمَمَاتِى لِلَّهِ رَبِّ ٱلْعَٰلَمِينَ 

Say, "Indeed, my prayer, my rites of sacrifice, my living and my dying are for Allāh, Lord of the worlds.

A muslim should not slaughter for anyone except Allah as slaughtering is an act of worship. 

Allah says, 

Al-Hajj 22:37

Their meat will not reach Allāh, nor will their blood, but what reaches Him is piety from you. Thus have We subjected them to you that you may glorify Allāh for that [to] which He has guided you; and give good tidings to the doers of good.

May Allah guides us and our eeman. Aamin