Praise be to Allah.
This hadeeth is to be found in Saheeh Muslim (2767), narrated from Abu Moosa (may Allaah be pleased with him) from the Prophet (peace and blessings of Allaah be upon him), who said: “On the Day of Resurrection, some of the Muslims will come with sins like mountains, but Allaah will forgive them and will put them (the sins) onto the Jews and Christians.” So this hadeeth is saheeh.
With regard to its meaning, al-Nawawi said in his commentary: “The meaning of this hadeeth is the same as the hadeeth narrated by Abu Hurayrah: “For each person there is a place in Paradise and a place in Hell.” When the believer enters Paradise, his place in Hell is taken over by the kaafir because he deserves that because of his kufr. The meaning of the phrase “your ransom from the Fire” is that you could have entered Hell, but this is your ransom because Allaah has decreed a certain number to fill it; when the kuffaar enter Hell because of their kufr and sins, this is like a ransom for the Muslims. With regard to the report, “On the Day of Resurrection, some of the Muslims will come with sins” – Allaah will forgive the Muslims these sins and will absolve them of them, and He will put the like of those sins onto the Jews and Christians because of their kufr and sins. So they will enter Hell because of their own actions, not because of the sins of the Muslims. This report has to be interpreted in this manner, because Allaah says (interpretation of the meaning):
“And no bearer of burdens shall bear another’s burden” [Faatir 35:18]
It may be that the meaning is: sins which the kuffaar innovated and initiated, because whoever starts a bad thing will bear a burden of sin like that of each person who does that thing.