It is narrated from Sayyiduna Ali (may Allah be pleased with him), he says: We used to think that by Salat al-Wusta was meant the Fajr prayer, but the Messenger of Allah (peace and blessings be upon him) said: It is the Asr prayer. By Salat al-Wusta, he (peace and blessings be upon him) meant the Asr prayer.
Hadith Referenceالفتح الربانی / كتاب الصلاة / 1137