Musnad Ahmad

It is narrated from Hazrat Qateelah (may Allah be pleased with her) that a great scholar from the People of the Book came to the presence of the Prophet and said, "O Muhammad! You would be the best nation if you did not commit shirk (associate partners with Allah)." The Prophet (peace and blessings be upon him) said, "Subhan Allah! How is that?" He said, "You swear by the Ka'bah when taking oaths." The Prophet (peace and blessings be upon him) remained silent for a while and then said, "He is speaking the truth, so from now on, whoever swears an oath should swear by the Lord of the Ka'bah." Then he said, "O Muhammad! If you did not associate anyone with Allah, you would be the best nation." The Prophet (peace and blessings be upon him) said, "Subhan Allah! How is that?" He said, "You say, 'What Allah willed and you willed.'" The Prophet (peace and blessings be upon him) remained silent for a while and then said, "He is speaking the truth, so whoever says this should make a distinction between the two phrases."