Musnad Ahmad 7:7

Yazid narrated to us, Ibn Abi Dhi'b informed us, from Muslim ibn Jundub, from al-Zubayr ibn al-'Awwam, who said: "We used to pray Jumu'ah with the Prophet, may Allah bless him and grant him peace, then we would depart and hasten toward the *ajam* (thickets/fortresses), and we would not find [space] except for the size of where our feet stood." Yazid said: "The *ajam* are the *atam* (fortresses/towers)."