Musnad Ahmad 26:834

Ya'qub narrated to us, my father narrated to us, from Ibn Ishaq, Muhammad ibn Ka'b al-Qurazi narrated to us, from the one who narrated to him, from Abdullah ibn Mas'ud, who said: While we were with him on a Friday in the mosque of Kufa, and Ammar ibn Yasir was the governor of Kufa for Umar ibn al-Khattab, and Abdullah ibn Mas'ud was in charge of the public treasury, Abdullah ibn Mas'ud looked at the shadow and saw it was the length of a sandal strap. He said: "If your companion follows the Sunnah of your Prophet, may Allah bless him and grant him peace, he will come out now." He said: By Allah, Abdullah ibn Mas'ud had not finished his speech before Ammar ibn Yasir came out saying: "The prayer."