It is narrated from Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "In Paradise there is a gate called al-Rayyan; none will enter through it except those who fast. When they have all entered through it, it will be closed."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5805