Musnad Ahmad 2:288

Hajjaj narrated to us, Ibn Lahi'ah informed us, from Abdullah ibn Hubayrah, from Abu Tamim, that he heard Umar ibn al-Khattab saying: I heard the Prophet, peace and blessings of Allah be upon him, saying: "If you were to rely upon Allah with the true reliance due to Him, He would provide for you just as He provides for the birds; they go out in the morning hungry, and return in the evening with full bellies."