It is narrated from the Mother of the Believers, Sayyidah Aishah (may Allah be pleased with her), that the Messenger of Allah (peace and blessings be upon him) said: "Whoever prays Fajr and remains sitting in his place of prayer until the sun rises, then prays four rak'ahs, Allah Almighty forgives his sins."
Hadith Referenceالمعجم الأوسط / باب الميم / 5940
Hadith Takhrij«أخرجه أبو يعلى فى «مسنده» برقم: 4365، وابن حجر فى «المطالب العالية» برقم: 655، 3397»