Musnad Ahmad 1153:2178

'Abd al-Rahman ibn Mahdi narrated to us, he said: Yazid ibn Ibrahim narrated to us, from Ibn Abi Mulaykah, from al-Qasim ibn Muhammad, from 'Aishah, that the Prophet, peace and blessings of Allah be upon him, recited this verse: "It is He who has sent down to you, [O Muhammad], the Book; in it are verses [that are] precise - they are the foundation of the Book - and others unspecific. As for those in whose hearts is deviation [from truth], they will follow that of it which is unspecific, seeking discord and seeking an interpretation [suitable to them]. And no one knows its [true] interpretation except Allah" [Al 'Imran: 7]. Then the Messenger of Allah, peace and blessings of Allah be upon him, said: "So if you see those who follow what is unspecific of it, then those are the ones whom Allah has named - or he understood - so beware of them."