Musnad Ahmad 27:1111

Yazid narrated to us, Hisham al-Dastawa'i informed us, from Yahya ibn Abi Kathir, from Abu Salam, from al-Hakam ibn Mina, that Ibn Umar and Ibn Abbas narrated that they heard the Messenger of Allah, peace and blessings of Allah be upon him, saying upon the planks of the pulpit: "People must cease neglecting the Friday prayers, or Allah will surely seal their hearts, and they will surely be written among the heedless."