Supplicating to someone than Allah is shirk

Islam is only religion that is completely monotheistic and unitarian. Islam teaches us that Allah is the Creator of everything and that He is All-Powerful, All-Knowing and All-Wise. Nothing happens except what He has willed. It was religion of all Prophets and Messengers peace be upon them who were sent by Allah to preach the message of Tawheed to their nations.


These nations used to worship idols which were made by their hands, they would name them after righteous people. They thought that these idols would bring them closer to Allah, so they dedicated their acts of worship to them instead of Allah. Allah has said that these idols are useless and condemned their worshippers as disbelievers.


He says in Qur’an:


And they worship besides Allāh that which does not possess for them [the power of] provision from the heavens and the earth at all, and [in fact], they are unable. (16:73)


Islam came to get rid of these disbeliefs and teaches us that Allah is worthy of being worshipped as He has created all of us and provided us everything, and He has full control of everything that exists. So all acts of worship must be submitted to Him and not anyone else otherwise those who have done so will commit shirk which puts them out of fold of Islam.


Supplication is one of acts of worship. Allah says:


Say, "Call upon Allāh or call upon the Most Merciful [ar-Raḥmān]. Whichever [name] you call - to Him belong the best names." And do not recite [too] loudly in your prayer or [too] quietly but seek between that an [intermediate] way. (17:110)


Islam has removed the concept of intermediation. Unlike Christianity, the Muslim dos not need to invoke anyone for asking Allah. Instead they should directly supplicate to Him as He says:


And when My servants ask you, [O Muḥammad], concerning Me - indeed I am near. I respond to the invocation of the supplicant when he calls upon Me. So let them respond to Me [by obedience] and believe in Me that they may be [rightly] guided. (2:186)


In fact Allah gets angry with those who do not call Him


And your Lord says, "Call upon Me; I will respond to you." Indeed, those who disdain My worship will enter Hell [rendered] contemptible. (40:60)


Prophet peace be upon him said:


“The supplication, is worship.” (Tirmidhi 3372)
Ibn 'Abbas (RAA) narrated, ‘One day I was riding behind the Prophet when he said, “O lad, be mindful of Allah and He will protect you. Be mindful of Allah and you shall find Him with you. When you ask (for anything), ask it from Allah, and if you seek help, seek help from Allah.” Related by At-Tirmidhi who verified it as Hasan and Sahih. (Bulguh al-Maram 1472)
To supplicate to anyone than Allah is shirk, regardless of their status such as Prophets, angels, jinns or any pious people such as Hussain bin Ali (or anyone from Ahle-Bayt), it is same as calling idols for help.