It is narrated from Sayyiduna Abu Umar al-Ansari (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever prays four rak'ahs before Zuhr, its reward is equal to freeing a slave from the descendants of Isma'il."
Hadith Referenceالمعجم الكبير للطبراني / مسند من يعرف بالكنى / 19168