Musnad Ahmad 1016:16

Abdullah said: I found in my father’s book, in his own handwriting: Muhammad ibn Bakr narrated to us, Ibn Jurayj informed us, Ja’far ibn Muhammad informed me, from his father, from Abdullah ibn Malik ibn Buhaynah, that the Prophet, peace and blessings of Allah be upon him, went out for the morning prayer, and Ibn al-Qashab was praying. The Prophet, peace and blessings of Allah be upon him, struck his shoulder and said: "O Ibn al-Qashab, are you praying the morning prayer as four [rak'ahs], or twice?" Ibn Jurayj is in doubt.