(a) Imam Malik (may Allah have mercy on him) says that it reached him from Sayyiduna Ali bin Abi Talib and Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that they both held that the middle prayer means the morning prayer.
(b) Imam Malik (may Allah have mercy on him) says: This is my view.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2168