A prophet says: Allah puts the sins of Muslims on the Jews and the Christians!

They said sarcastically: Your prophet's mercy reached its maximum level when he said: "On the day of judgment, some Muslims will come with sins like mountains, then Allah forgives them and puts their sins on the Jews and the Christians".
Is that mercy?! The speech is mentioned in The Correct Muslim, book of (Repentance), chapter (Accepting a Killer's Repentance Even if He Killed Many Times), no. 4971. Ghailan Ibn Jarir narrated from Abu Burda, narrated from his father that the prophet (PBUH) said: "On the day of judgment, some Muslims will come with sins like mountains, then Allah forgives them and puts their sins on the Jews and the Christians". (As far as I think), Abu Raub said: I do not know as to who is in doubt. Abu Burda said: I narrated it to 'Umar b. 'Abd al-'Aziz, whereupon he said: Was it your father who narrated it to you from Allah's Apostle (may peace be upon him)? I said: Yes.