Narrated Abu Sa’eed (radiyallaahu-anhu): I heard the Prophet (sallal-laahu-alayhi-wasallam) saying, ”Whoever observes Sawm (fast) for one day in Allaah’s Cause (to seek His good pleasure), Allaah will keep his face away from (hell) fire (a distance covered by a journey of ) seventy years”.
[Sahih al-Bukhaari. Vol 4. Hadeeth number 2840. Darussalam Print]