Musannaf Ibn Abi Shayba 21:126

Yahya ibn Adam narrated to us, he said: Ammar ibn Ruzayq narrated to us, from Abu Ishaq, from Abu Burdah, who said: "The hour in which the supplication of one who calls upon [Allah] is answered on the day of Jumu'ah is when the Imam stands for the prayer until he finishes it."