Musnad Ahmad 1469

Ibraheem- i.e., bin Sa"d narrated that Mu"adh at-Taimi said: I heard Sa"d bin Abi Waqqas say: I heard the Prophet (ﷺ) say: "There are two prayers after which no nafl prayer is to be offered: Fajr, until the sun rises and "Asr until the sun sets.”