Musnad Ahmad 1007:119

Abu Sa'id narrated to us, Harb narrated to us, Yahya narrated to us, from Abu Salamah, from Abu Qatadah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever sees a dream that pleases him, let him speak of it, for it is glad tidings from Allah. And whoever sees a dream that he dislikes, let him not speak of it, and let him spit to his left, and seek refuge in Allah from its evil."