The intention behind accepting Islam and performing all acts of worship must be purely devoted to Allah (عز و جل ). The evidence of sincerity which negates shirk, is Allah (عز و جل )’s saying:

“Surely, the religion is for Allah only…” [Az-Zumar 39:3]

And:

“And they were commanded not, but that they should worship Allah, and worship none but Him Alone (abstaining from ascribing partners to Him), and perform As-Salat (Iqamat-as-Salat) and give Zakat: and that is the right religion.” [Al-Baiyinah 98:5]

It is narrated from Abu Hurairah (رضي الله عنه ) that the Prophet (صلى الله عليه و سلم ) said: “The happiest person who will have my intercession will be the one who said sincerely (from the bottom of his heart): None is worthy of worship except Allah.” [Al-Bukhari]

Also narrated by Otban bin Malik (رضي الله عنه ) that Prophet (صلى الله عليه و سلم ) said:
“Allah has forbidden hell for those who say: (there in no ilah worthy of worship except Allah) for the sake of Allah only.” [Al-Bukhari]

Referring to the hadith quoted earlier “The happiest person…” this hadith is reported by Imam Al-Bukhari in his Sahih and others. It is also narrated by Abu Hurairah (رضي الله عنه ) who said:
“It was said: “O Messenger of Allah (صلى الله عليه و سلم ); who is the happiest of mankind with your intercession on the Day of Judgement? The Messenger of Allah (صلى الله عليه و سلم ) said: I thought, O Abu Hurairah that no one would ask me about this hadith before you because of what I have witnessed of your eagerness for hadith. The happiest of mankind with my intercession on the Day of Judgement is the one who said sincerely (from the bottom of his heart) none is worthy of worship except Allah.” [Reported by Al-Bukhari]