Shu'aib bin Shu'aib bin Ishaq شعيب بن شعيب بن إسحاق · The Book of Fasting

"The Messenger of Allah said: Whoever fasts a day in the cause of Allah, Allah will separate his face (a distance of) seventy autumns from the fire in return for that day."

That the Messenger of Allah , Abu Bakr and 'Umar were in Marr Az-Zahran. He narrated it in Mursal from. (Daif)