Musannaf Ibn Abi Shayba 3:4972

Affan narrated to us, he said: Abu Awanah narrated to us, he said: Ibrahim ibn Muhammad ibn al-Muntashir narrated to us, from his father, that he used to pray two rak'ahs after the Asr prayer. It was said to him [about that], and he said: "Even if I did not pray them except that I saw Masruq praying them, it would be sufficient [as a reason], but I asked Aisha, and she said: 'The Messenger of Allah, may Allah bless him and grant him peace, would not leave two rak'ahs before the Fajr and two rak'ahs after the Asr.'"