Musnad Ahmad 30:3677

Abdullah ibn Yazid narrated to us, Ibn Lahi'ah narrated to us, from Khalid ibn Yazid, from Lahi'ah Abu Abdullah, from a man whom he named, from Salamah ibn Qaysar, from Abu Hurairah, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts a day seeking the Face of Allah the Almighty and Majestic, Allah distances him from Hell as the distance of a crow that flew while it was a fledgling until it died of old age."