In the khutbah today, the shaykh mentioned that there is no specific hadith that mentions that the rewards of deeds are multiplied in Ramadan -- except for one, which is da'eef.
He mentioned that there is a difference of opinion about multiplication of rewards in Ramadan; and the scholars who say that it multiplies, use some sort of qiyas. They use proofs such as the hadith about praying in ka'bah is 100,000 times praying anywhere else to establish that multiplication of rewards is possible.
So is there a specific, authentic hadith that mentions that rewards of deeds are multiplied in Ramadan?