It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "A man was praying, and when he prostrated, another man came to him and stepped on his neck. The one who was beneath said: By Allah, he will never be forgiven. Allah, the Mighty and Majestic, said: My servant has sworn by Me that I will not forgive My other servant, whereas I have forgiven him."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10050