Musnad Ahmad 70:2

Hammad ibn Khalid al-Khayyat narrated to us, Amr ibn Kathir ibn Aflah narrated to us, from Abd al-Rahman ibn Kaysan, who said: I asked my father, Kaysan, "What did you witness of the Prophet, may Allah bless him and grant him peace?" He said: "I saw him praying at the upper well, the well of Banu Muti', with his garment wrapped around him, during the Dhuhr or Asr prayer, and he prayed it as two rak'ahs."