Mukhtasar Bukhari

Umm al-Mu’minin Aisha Siddiqa (may Allah be pleased with her) says: By the One who took the Noble Prophet (peace be upon him) from this world, the Prophet (peace be upon him) never abandoned the two rak’ahs after Asr until he met Allah, and when he (peace be upon him) met Allah, at that time (due to the heaviness of the body) he (peace be upon him) would become burdened from prayer, and he (peace be upon him) would perform many of his prayers sitting, and the Noble Prophet (peace be upon him) would always perform these two rak’ahs, that is, the two rak’ahs after Asr, and he would perform them at home, out of fear that it might become burdensome for his Ummah. The Prophet (peace be upon him) liked only that which would be easy for his Ummah.

Umm al-Mu'mineen Aisha Siddiqa (may Allah be pleased with her) says that the Messenger of Allah (peace and blessings be upon him) never abandoned two units of prayer, whether in secret or in public: two units before the Fajr prayer and two units after the Asr prayer.