Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: Once I came to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was sitting on a raised place. I sat in front of him (peace and blessings be upon him), my face being level with the knees of the Prophet (peace and blessings be upon him). I took advantage of the Prophet's (peace and blessings be upon him) being alone and asked: O Messenger of Allah! Which sin is the greatest? The Prophet (peace and blessings be upon him) turned away from me, until I had repeated my question three times; then he (peace and blessings be upon him) turned towards me. After this the narrator mentioned the whole hadith.
Al-Bazzar recorded it; its chain contains a narrator, al-Sari bin Isma'il, and he is matruk (abandoned).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب العلم / 731