Musnad Ahmad 1013:24

'Affan narrated to us, Bishr ibn al-Mufaddal narrated to us, 'Abd al-Rahman ibn Ishaq narrated to us, from Abu Hazim, from Sahl ibn Sa'd, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, for Paradise there is a gate called al-Rayyan. It will be said on the Day of Resurrection: 'Where are the fasting ones?' So when they have entered it, it will be closed, and no one else will enter through it." He said: So I met Abu Hazim and asked him, and he narrated it to me, except that I am more mindful of the hadith of 'Abd al-Rahman.