Musnad Ahmad 1153:155

Jarir narrated to us, from Qabus, from his father, who said: My father sent a woman to Aisha to ask her: Which prayer was most beloved to the Messenger of Allah, may Allah bless him and grant him peace, for him to persist in? She said: "He used to pray four [rak'ahs] before the noon prayer, in which he would lengthen the standing, and perfect the bowing and the prostration. As for that which he would never abandon—neither when healthy, nor when ill, nor when absent, nor when present—it was two rak'ahs before the dawn."