Musnad Ahmad 1:68

Sufyan narrated to us, he said: Ibn Abi Khalid narrated to us, from Abu Bakr ibn Abi Zuhayr—I think he said: Abu Bakr said: "O Messenger of Allah, how is righteousness [attained] after this verse?" He said: "May Allah have mercy on you, O Abu Bakr. Do you not fall ill? Do you not grieve? Do you not suffer hardship? Do you not..." He said: "Yes." He said: "Then that is for that."