Musnad Abu Ya'la al-Mawsili 6:104

Ibrahim ibn al-Hajjaj al-Sami narrated to us, Abd al-Wahid narrated to us, al-A'mash narrated to us, from Malik ibn al-Harith. Al-A'mash said: And I heard them mentioning it from Mus'ab ibn Sa'd, from his father, and I do not know them except that they mentioned it from the Prophet, may Allah bless him and grant him peace, who said: "Deliberation is good in everything, except in the work of the Hereafter."