Sayyiduna Safwan (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever prays four rak'ahs before Zuhr, it will be for him like freeing ten slaves (the narrator is unsure; perhaps the words are:) like freeing four slaves from the descendants of Sayyiduna Isma'il (peace be upon him)."
Al-Tabarani recorded it in al-Mu'jam al-Awsat; its chain contains a group of narrators whose biographies I have not found (written in any book).