مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3573
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: A man came to the Prophet (ﷺ) and said: O Messenger of Allah, if I do well in Islam, will I be blamed for what I did during the Jahiliyyah? He said: "If you do well in Islam, you will not be blamed for what you did during the Jahiliyyah, but if you do badly in Islam, you will be blamed for the former and the latter."
Musnad Ahmad 3596Its isnad is saheeh, al-Bukhari (6921) and Muslim (120)]