On the Day of Resurrection, my Ummah will be gathered into three groups
Abu Musa (radi Allahu anhu) narrated that the Prophet (sal Allahu alaihi wa sallam) said, “On the Day of Resurrection, my Ummah (nation) will be gathered into three groups, one (group) will enter Paradise without rendering an account (of their deeds). Another (group) will be reckoned on easy account and admitted into Paradise. Yet another (group) will come bearing on their backs heaps of sins like great mountains. Allah will ask the angels though He knows best about them: Who are these people? They will reply: They are humble slaves of yours. He will say: Unload the sins from them and put the same over the Jews and Christians; then let the humble slaves enter Paradise by virtue of My Mercy.”
[Hadith Hasan - Mustadrak of Hakim]
Another similar narration found in Sahih Muslim and also narrated from Abu Musa (radi Allahu anhu) from the Prophet (sal Allahu alaihi wa sallam), who said: “On the Day of Resurrection, some of the Muslims will come with sins like mountains, but Allah will forgive them and will put them (the sins) onto the Jews and Christians.”
With regard to its meaning, Imam al-Nawawi (may Allah have mercy upon him) said in his commentary: “The meaning of this Hadith is the same as the Hadith narrated by Abu Hurayrah (radi Allahu anhu): “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.
With regard to the report, “On the Day of Resurrection, some of the Muslims will come with sins” – Allah 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 Allah 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.
And Allah knows best!