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.
Indeed it is confirmed in the texts of the Qur'aan and the Sunnah of the Prophet that in principle the reward is multiplied. Some rewards are multiplied tenfold as Allaah Says (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]. Some other rewards are multiplied 700 times as Allaah Says (which means): {The example of those who spend their wealth in the way of Allaah is like a seed [of grain] which grows seven spikes; in each spike is a hundred grains. Allaah multiplies [His reward] for whom He wills. And Allaah is All-Encompassing and All-Knowing.}[2:261]. However, none knows how many times some other rewards are multiplied but Allaah. Allaah Says (which means): {Who is it that would loan Allaah a goodly loan so that He may multiply it for him many times? And it is Allaah who decreases or increases your provisions, and unto Him you shall return.}[2:245]. So it is only Allaah Who knows how many times the reward is multiplied and indeed Allaah's Favour and Grace are vast. As regards the month of Ramadan, we are not aware of any authentic narration of the Prophet which proves exactly how many times the good deeds are multiplied except one weak narration which is reported by Al-Bayhaqi and Ibn Al-'Asaakir. The narration reads: "Whoever does a prescribed act of worship during this month it is as if he had performed 70 acts in other months."
This narration was classified as weak by Ibn Hajar and as weak and fabricated by Al-Albaani .
Regarding the prayer in the Madeenah mosque, its reward is multiplied 1000 times compared to the prayer in other mosques and the prayer in Al-Haram mosque (i.e. Makkah) is multiplied 100,000 times (the reward of prayer in other mosques), as in the narration reported by At-Tabraani and others.
Allaah knows best.