مُسْنَدُ أَحْمَدَ
Musnad Ahmad 4105
Musnad of Abdullah bin Mas'ood
Wasil al-Ahdab said: I heard Abu Wa'il say: 'Abdullah said: I asked the Messenger of Allah (ﷺ): Which sin is worst...? And he narrated it.
Musnad Ahmad 4132Its isnad is saheeh, al-Bukhari (4761) and Muslim (86)]