Musnad Ahmad 435:87

Hajjaj narrated to us, he said: Shu'bah narrated to us, from Abu al-Tayyah, he said: I heard Humran ibn Aban narrating from Mu'awiyah, that he saw people praying after the 'Asr [prayer]. He said: "You are indeed performing a prayer that we accompanied the Prophet, may Allah bless him and grant him peace, and we never saw him perform it, and he indeed forbade it," meaning the two rak'ahs after 'Asr.