Imam Qurthubi says: The reward of all good deeds are disclosed before men .i.e It will be ten to seven hundred times .But in the case of Fasting, Allah gives rewards without limits.We see in one Hadith as follows;All the good deeds of the Son of Adam will be rewarded ten times to seven hundred times as Allah wills.But Allah says…''except Fasting;certainly Fasting is for me and It is me who reward for it.This means that Allah will give the rewards for Fasting abundantly without limits.This is clear from the following verse of Holy Quran.
إِنَّمَا يُوَفَّى الصَّابِرُونَ أَجْرَهُمْ بِغَيْرِ حِسَابٍ
Only those who are patient shall receive their reward in full, without reckoning
Here , most of the Scholars say that 'saaboroon/those whoare patient' means Fasting persons.Please see the full text and translation of Az-zumar verse 10
قُلْلْ يَا عِبَادِ الَّذِينَ آمَنُوا اتَّقُوا رَبَّكُمْ لِلَّذِينَ أَحْسَنُوا فِي هَذِهِ الدُّنْيَا حَسَنَةٌ وَأَرْضُ اللَّهِ وَاسِعَةٌ إِنَّمَا يُوَفَّى الصَّابِرُونَ أَجْرَهُم بِغَيْرِ حِسَابٍ
Say: "O My servants who believe, have Taqwa of your Lord. Good is for those who do good in this world, and Allah's earth is spacious! Only those who are patient shall receive their reward in full, without reckoning