What is the ruling on saying "so you know that Allah is truth" to someone who erred after supplicating against them with a calamity?

There is no problem with the idea that the answering of prayers is proof of God's existence and His truth. A person supplicates, and God answers him, through which we come to know God's existence, His power, His mercy, and His truthfulness. Ibn Aqil said that supplication indicates God's existence, His self-sufficiency, His hearing, His generosity, His mercy, and His power, and that one who is not thus should not be supplicated. Al-Uthaymeen said that the answering of prayers is proof of God's existence, His power, His mercy, and His truthfulness, citing His Almighty's saying: "Call upon Me; I will respond to you." No scholar has forbidden this statement despite its prevalence.