Musnad Abu Ya'la al-Mawsili 155:182

Abu Abd al-Rahman al-Adhrami narrated to us, Zayd ibn al-Hubab narrated to us, from Umar ibn Abi Khath'am al-Yamami, from Yahya ibn Abi Kathir, from Abu Salamah, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever prays six rak'ahs after the Maghrib [prayer], and does not speak any evil between them, it is equivalent for him to the worship of twelve years."