Musnad Ahmad 28:545

Ubaydah ibn Humayd Abu Abd al-Rahman narrated to us, Ata’ ibn al-Sa’ib narrated to me, from his father, from Abdullah ibn Amr, who said: The Messenger of Allah (ﷺ) said: "O Abdullah ibn Amr, in how much time do you recite the Quran?" He said: I said: "In my day and my night." He said: So he said to me: "Sleep and pray, and pray and sleep, and recite it in every month." He said: I continued to reduce it for him, and he continued to reduce it for me, until he said: "Recite it in every seven nights." My father said: "And I did not understand, and a word escaped me." He said: Then he said: I said: "I fast and do not break my fast?" He said: So he said to me: "Fast and break your fast, and fast three days of every month." He said: I continued to reduce it for him, and he continued to reduce it for me, until he said: "Fast the most beloved fasting to Allah the Almighty, the fasting of David: fast one day and break your fast one day." Abdullah ibn Amr said: "That I had accepted the concession of the Messenger of Allah (ﷺ) is more beloved to me than having the red camels" — I think he doubted Ubaydah.