Abdullah bin Yasar (may Allah have mercy on him) narrates from Qutaylah bint Sayfi al-Juhaniyyah (may Allah be pleased with her) that a Jewish scholar came to the Prophet (peace and blessings be upon him) and said: O Muhammad! You would be excellent people were it not that you associate partners (with Allah). He (peace and blessings be upon him) said: "'Subhan Allah' (Glory be to Allah)! How is that?" He said: When you swear an oath, you say: By the Ka'bah. The Prophet (peace and blessings be upon him) paused, then said: "Whoever swears an oath, let him swear by the Lord of the Ka'bah." Then he said: You would be excellent people were it not that you say: Whatever Allah wills and so-and-so wills. The Prophet (peace and blessings be upon him) paused for a little while, then said: "Whoever says: 'Whatever Allah wills,' then let him say after a pause: 'then whatever you will.'"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5811