Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) narrates: "Three men sat by the door of the Ka'bah, two of Quraysh and one of Thaqif, or two of Thaqif and one of Quraysh. Some of them said something secretly to others, and one of them said: 'Do you think Allah hears what we say?' One of them said: 'What we say aloud, He certainly hears.' The other said: 'If He hears what is said aloud, then He also hears what is said secretly.' So Allah, the Blessed and Exalted, revealed this verse: 'And you were not concealing yourselves' [Surah Fussilat: 22], the complete verse."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10102