The Quran

أَلا لِلَّهِ الدّينُ الخالِصُ ۚ وَالَّذينَ اتَّخَذوا مِن دونِهِ أَولِياءَ ما نَعبُدُهُم إِلّا لِيُقَرِّبونا إِلَى اللَّهِ زُلفىٰ إِنَّ اللَّهَ يَحكُمُ بَينَهُم في ما هُم فيهِ يَختَلِفونَ ۗ إِنَّ اللَّهَ لا يَهدي مَن هُوَ كاذِبٌ كَفّارٌ

Muhammad Asad

Is it not to God alone that all sincere faith is due? And yet, they who take for their protectors aughtbeside Him [are wont to say], "We worship them for no other reason than that they bring us nearer to God."¹ Behold, God will judge between them² [on Resurrection Day] with regard to all wherein they differ [from the truth]: for, verily, God does not grace with His guidance anyone who is bent on lying [to himself³ and is] stubbornly ingrate!

This relates not only to the worship of saints, angels and "deified" persons as such, but also to that of their symbolic representations (statues, pictures, relics, etc.) and, in the case of defunct human personalities, of their real or reputed tombs. Since all such practices are based on the worshipper's hope of "mediation" between himself and God, they obviously conflict with the concept of His omniscience and justice, and are, therefore - notwithstanding their widespread occurrence - utterly rejected by the Qur'an.
I.e., between those worshippers and the spiritual leaders who have led them astray (cf. 34:31-33).
Cf. 6:22-24 and the corresponding notes.