Abu Hurairah reported,
He traced this back to the Prophet ﷺ, "Allah 
truly laughs at two men, one of whom killed the other, yet both enter Paradise together. He explained, one was a disbeliever who killed a Muslim, then the disbeliever embraced Islam before he died, so Allah admitted them both into Paradise."
Ahmad 7326
Sahihحدثنا سفيان عن أبي الزناد عن الأعرج
عن أبي هريرة
يبلغ به النبي ﷺ إن الله عز وجل ليضحك من الرجلين قتل أحدهما الآخر يدخلان الجنة جميعا يقول كان كافرا قتل مسلما ثم إن الكافر أسلم قبل أن يموت فأدخلهما الله عز وجل الجنة
No references are available.
Its isnad is sahih. Muslim narrated it, 2:99, at length, similarly, through Sufyan, with this isnad. Nasai narrated it, 2:63, through Sufyan, in a shortened form. Malik narrated it in Muwatta, 460, similarly to the version in al-Musnad, from Abu Zinad from Aaraj. Bukhari narrated it, 6:29-30, through Malik. Muslim also narrated it, from the hadith of Hammam bin Munabbih, from Abu Hurairah.