Musnad Ahmad 25:294

Yazid narrated to us, Hisham al-Dastawa'i informed us, from Yahya ibn Abi Kathir, from Abu Salam, from al-Hakam ibn Mina, from Ibn Umar and Ibn Abbas, that they both testified regarding the Messenger of Allah (may Allah bless him and grant him peace) that he said while he was upon the steps of the pulpit: "People must cease neglecting the Friday prayers, or Allah the Almighty and Majestic will surely seal their hearts, and they will surely be written among the heedless."