Musnad Ahmad 8:6

Rawh narrated to us, dictating it to us in Baghdad, Muhammad ibn Abi Humayd narrated to us, from Isma'il ibn Muhammad ibn Sa'd ibn Abi Waqqas, from his father, from his grandfather Sa'd ibn Abi Waqqas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Among the happiness of the son of Adam is his seeking guidance from Allah (Istikhara), and among the happiness of the son of Adam is his contentment with what Allah has decreed. And among the wretchedness of the son of Adam is his abandoning seeking guidance from Allah, and among the wretchedness of the son of Adam is his discontent with what Allah, the Almighty and Majestic, has decreed."