All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
We do not know an authentic narration from the Sunnah which indicates that the reward of fasting is multiplied 700 times. Rather, it is confirmed that Allaah rewards for fasting without limitation. Imaam Muslim reported that Abu Hurayrah said: "The Prophet said: "The reward of every act of the son of Aadam will be multiplied tenfold up to 700 times, Allaah said except fasting, (it is done for My Sake and) I will reward for it (as much as I wish)." Imaams Al-Qurtubi, Ibn Al-Jawzi and others said that the narration means that the reward for the acts of good deeds of people is known to them and that they are multiplied tenfold up to 700 times except fasting, Allaah rewards for it without limitation. However, people differ in the degree of this reward they will receive. Some of them get the reward stated, and some get a reward less than that, and some others do not get anything of this great excellence and favour, based upon the extent of their fulfilling the conditions and etiquettes of fasting. The Prophet said: "Many a fasting person does not get from his fasting except hunger." [Ahmad, Ibn Maajah and others] So no one knows that he has indeed obtained this great favour. The above is about fasting. As regards the multiplication of the reward of the prayer, it is included in the saying of Allaah (which means): {Whoever comes [on the Day of Judgement] with a good deed will have ten times the like thereof [to his credit]}[6:160]. But it could be multiplied by more than this; verily Allaah grants His Favours upon whomever He wants. However, it should be noted that the reward of fasting and praying being multiplied does not mean that if one misses an obligation he does not have to make it up. Whoever misses an obligatory prayer or obligatory fasting he has to make them up because an obligation must be made up if missed.
Allaah knows best.