The Messenger of Allah (sal Allahu alaihi wa sallam) said: “Allah said, ‘My word does not change, as it is enjoined upon you in Umm al-Kitaab, every Hasanah will have the reward of ten like it, so they are fifty in Umm al-Kitaab and five enjoined upon you.” [Bukhaari]
Our Messenger Muhammad (sal Allahu alaihi wa sallam) told us in the Hadith of the Israa (ascension to heaven) how he went back and forth between his Lord and Moosa (alaihis salam), when each time Moosa (alaihis salam) told him to go back to his Lord and ask Him to reduce the number of prayers required, until they became five instead of fifty.
So he went back to Moosa (alaihis salam), who said: “What did you do?” He said: “It is reduced for us, and for every Hasanah we will be given the reward of ten like it.” [Bukhaari]
Moosa (alaihis salam) from his experience with the Bani Israeel knew how difficult people were, how ungrateful to their Lord, and how easily misled by Shaitaan. So he kept advising Muhammad (sal Allahu alaihi wa sallam) to ask Allah (subhana wa ta’ala) to reduce the number. This continued till Muhammad (sal Allahu alaihi wa sallam) felt too shy to ask Allah (subhana wa ta’ala) any more.
Allah (subhana wa ta’ala) in His generosity promised to still give us the reward of fifty prayers if we just prayed five times a day. Each prayer is rewarded as ten prayers. What a shame that despite this incentive we cannot pray even five prayers a day!