Musnad Ahmad 25:452

'Affan narrated to us, Aban al-'Attar narrated to us, Yahya ibn Abi Kathir narrated to us, from Zayd, from Abu Salam, from al-Hakam ibn Mina', from Ibn 'Abbas and from Ibn 'Umar, that they both heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "People must cease neglecting the Friday prayers, or Allah will surely seal their hearts, and then they will surely be written among the heedless."