The Battle of Badr
3:121 Recall that you (Muhammad) were among your people when you set out to assign to the believers their positions for battle. GOD is Hearer, Omniscient.
3:122 Two groups among you almost failed, but GOD was their Lord. In GOD the believers shall trust.
3:123 GOD has granted you victory at Badr, despite your weakness. Therefore, you shall observe GOD, to show your appreciation.
http://en.wikipedia.org/wiki/Battle_of_Badr
How do you show your appreciation?
2:185 Ramadan is the month during which the Quran was revealed, providing guidance for the people, clear teachings, and the statute book. Those of you who witness this month shall fast therein. Those who are ill or traveling may substitute the same number of other days. GOD wishes for you convenience, not hardship, that you may fulfill your obligations, and to glorify GOD for guiding you, and to express your appreciation.