Hadith Qudsi 13

13) Narrated Jundub  that Allah's Messenger stated that a person said: By Allah! Allah will not forgive so-and-so (person). Thereupon Allah, the Exalted, remarked: Who is he who swears by Me (and says) that I will not forgive so-and-so (listen!) Verily I have forgiven so-and-so and nullified your deeds, or as the Messenger  said.
(This Had'ith is sound and reported by Muslim and
also mentioned in Mu'jam At-Tabriini AI-Kab'ir).
 

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This Hadith teaches us the politeness needed by a Muslim towards Allah  and that one should not involve himself in matters that are left for Allah  to decide i.e. nobody can know who will go to Hell and who will enter Paradise.