Shaykh Al-Islam Ibn Taymiyyah (رحمه الله تعالى) said:

“Whoever places an intermediary between himself and Allah, by making Duā’ to them, or asking them [for something], or has Tawakkul on them, has committed Kufr by consensus – and this is the Madhhab of [Imam] Ahmad according to all of his companions.”

[Majmū Al-Fatāwah 1:124 & 126]