Musannaf Ibn Abi Shayba 3:3575

Abu Bakr narrated to us, he said: Jarir ibn Abd al-Hamid narrated to us, from Qabus, from his father, who said: My father sent me to Aisha, asking: "Which prayer was most beloved to the Messenger of Allah, may Allah bless him and grant him peace, for him to be constant in?" She said: "He used to pray four [rak'ahs] before the Dhuhr [prayer], in which he would lengthen the standing and perfect the bowing and the prostration."