Musannaf Ibn Abi Shayba 3:5168

Mu'adh ibn Mu'adh narrated to us, he said: Ibn 'Awn informed us, from Nafi', who said: It reached 'Umar ibn al-Khattab that people were going to the tree under which the pledge of allegiance was taken. He said: So he ordered it, and it was cut down.