Sayyidah Qutaylah bint Sayfi (may Allah be pleased with her) said: One of the rabbis came to the Messenger of Allah (peace and blessings be upon him) and said: O Muhammad, you would be an excellent people if you did not commit shirk. The Prophet (peace and blessings be upon him) said: "How so?" He said: When you swear an oath, you say: "By the Ka'bah." The narrator said: The Messenger of Allah (peace and blessings be upon him) remained silent for a while, then said: "Whoever swears an oath, let him swear by the Lord of the Ka'bah." Then he said: O Muhammad, you would be an excellent people if you did not set up partners with Allah. He (peace and blessings be upon him) said: "Subhan Allah! How so?" He said: You say to a man: "Whatever Allah wills and you will." The narrator said: The Messenger of Allah (peace and blessings be upon him) remained silent for a while, then said: He has indeed said (this), so whoever says "Whatever Allah wills" should put a separation between the two and say "then you will."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21468