Du’a is the greatest act of worship.
The Prophet (saw) said, “It is the essence of worship.” (Abu Dawud and others)
“So pray unto Him, devoting your worship wholly and sincerely to Him.” (Ghaafir 40:65)
“And the places of worship are only for Allah, so pray not unto anyone along with Allah.” (Al-Jinn, 72:18)
“Say: I only call upon my Lord, and I do not associate any one with Him.” (Al-Jinn, 72:20)
“O men. A similitude is set forth, so listen to it. Surely those whom you call upon instead of Allah cannot create even a fly, though they should all combine together for the purpose. And if the fly should snatch away anything from them, they cannot recover it there from. Weak, indeed, are both the seeker and the sought.” (Al-Hajj, 22:73)
The most common prohibition in the Quran is making dua to other than Allah, it is mentioned more than the word shirk in the Quran.
Du’a is the greatest manifestation of worship. It is common to all religions. If it is the most common act of worship, it therefore follows it would be the most common form of shirk, and it is therefore prohibited most in the Quran.
“If you call on them they shall not hear your call, and even if they could hear they shall not answer you; and on the resurrection day they will deny your associating them (with Allah); and none can inform you like the One Who is Aware.” (Faatir, 35:14)
When du’a is made at the time of need and distress, all three pillars of ibaadah are combined.
When you make dua to other than Allah it implies,
That this deity loves you more than Allah.
That object can hear your du’a
That object can respond to your du’a
To make du’a to other than Allah is the greatest act of shirk. The entire message of the Quran is to make du’a to Allah alone.
“Surely, those whom you call on beside Allah are creatures like you. Call on them now, and let them answer you, if you are truthful. Have they feet with which they walk, or have they hands with which they hold, or have they eyes with which they see, or have they ears with which they hear? Say: Call your associates, then make a struggle (to prevail) against me and give me no respite. Surely my guardian is Allah, Who revealed the Book, and He protects the righteous. And they whom you call besides Him have no power to help you, nor can they even help themselves.” (Al-A’raaf, 7:194-197)
There is no act that better illustrates the relationship between the worshipper and the object he worships.