Mu'adh bin Mu'adh (may Allah have mercy on him) narrates from Ash'ath (may Allah have mercy on him) concerning Zuhr, and al-Hasan (may Allah have mercy on him) used to give the fatwa concerning Maghrib that the imam will have six rak'ahs and the followers three rak'ahs each.
Some people narrated it from Ash'ath concerning Maghrib as marfu' (attributed to the Prophet (peace and blessings be upon him)), and I can only think that he was mistaken in that.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الخوف / 6037