What is the ruling on one who believes that a wali (saint) or a guide has the power to grant children or to increase or decrease provisions?

Whoever believes that offspring or sustenance come from other than Allah, or that anyone other than Allah can increase or decrease sustenance, is guilty of a shirk (polytheism) more severe than the shirk of the Arabs in the Pre-Islamic Era of Ignorance. This is because they acknowledged that Allah is the Sustainer, and they only worshipped their false deities to bring them closer to Allah.

Therefore, giving and withholding are in the hands of Allah alone. Allah may grant His servant offspring and expand his sustenance due to his supplication and reliance on Him, as was narrated in the supplications of Ibrahim and Zakariya (peace be upon them). And it is also established in the Sunnah that maintaining ties of kinship increases sustenance and prolongs life.