Musnad Abu Ya'la al-Mawsili 161:23

Bishr ibn Sayhan narrated to us, Harb ibn Maymun narrated to us, Musa ibn Ubaydah al-Rabadhi narrated to us, from Muhammad ibn Ka’b al-Qurazi, from Abu Hurayrah, who said: I went out with the Messenger of Allah (may Allah bless him and grant him peace), and his hand was in my hand. He came upon a man of disheveled appearance. He said, "O so-and-so, what has brought you to the state I see?" He said, "Illness and hardship, O Messenger of Allah." He said, "Shall I not teach you words by which Allah will remove your illness and hardship?" He said, "Yes. It would not please me to have witnessed Badr and Uhud with you [in exchange for them]." The Messenger of Allah (may Allah bless him and grant him peace) laughed and then said, "And can the people of Badr and the people of Uhud attain what the contented poor person attains?" Abu Hurayrah said, "O Messenger of Allah, [teach] me, so teach me." He said, "Say, O Abu Hurayrah: 'I have relied upon the Ever-Living who does not die. Praise be to Allah, who has not taken a son, has had no partner in dominion, and has had no protector out of weakness, and glorify Him with [great] glorification.'" He said: Then the Messenger of Allah (may Allah bless him and grant him peace) came upon me, and my condition had improved. He said, "How are you?" I said, "O Messenger of Allah, I have not ceased saying the words you taught me."