Musnad Ahmad 31:68

Qutaybah ibn Sa'id narrated to us, Bakr ibn Mudar narrated to us, from Ibn al-Had, from Abdullah ibn Khabbab, from Abu Sa'id al-Khudri, that he heard the Messenger of Allah, peace and blessings of Allah be upon him, say: "If one of you sees a vision that he likes, it is only from Allah, so let him praise Allah for it and speak of it. But if he sees other than that, which he dislikes, it is only from Satan, so let him seek refuge in Allah from its evil, and let him not mention it to anyone, for it will not harm him."