Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever commits a sin and knows that Allah Almighty is aware of it, that person is forgiven, even if he has not sought forgiveness."
At-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a narrator, Ibrahim bin Harasah, who is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب التوبة / 17604