Musnad Ahmad 1200:1

Yahya ibn Sa'id narrated to us, he said: Al-Mas'udi narrated to us, he said: Ma'bad ibn Khalid narrated to me, from 'Abdullah ibn Yasar, from Qutaylah bint Sayfi al-Juhaniyyah, who said: A rabbi from the rabbis came to the Messenger of Allah, peace and blessings of Allah be upon him, and said: "O Muhammad, you are a good people, were it not that you commit shirk (polytheism)." He said: "Subhan Allah (Glory be to Allah), and what is that?" He said: "You say when you swear: 'By the Ka'bah'." She said: The Messenger of Allah, peace and blessings of Allah be upon him, paused for a while, then said: "He has spoken. So whoever swears, let him swear by the Lord of the Ka'bah." Then he said: "O Muhammad, you are a good people, were it not that you set up a rival to Allah." He said: "Subhan Allah, and what is that?" He said: "You say: 'What Allah wills and you will'." She said: The Messenger of Allah, peace and blessings of Allah be upon him, paused for a while, then said: "He has spoken. So whoever says 'What Allah wills', let him separate between them by saying: 'then what you will'."