Musnad Abu Ya'la al-Mawsili 6:13

Musa ibn Muhammad ibn Hayyan al-Basri narrated to us, Umar ibn Ali ibn Ata ibn Muqaddam narrated to us, from Abd al-Rahman ibn Abi Bakr ibn Ubayd Allah, from Ismail ibn Muhammad, from his father, from his grandfather, that the Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, among the happiness of a person is his seeking the choice of his Lord (Istikhara) and his contentment with what He has decreed. And indeed, the wretchedness of the servant is his abandoning the seeking of the choice of his Lord and his discontent with what He has decreed."