Musnad Abu Ya'la al-Mawsili 16:3

Abu Salamah ibn al-Sabbak narrated to us, Makhlad narrated to us, from Ibn Jurayj, from Ja'far ibn Muhammad, from his father, from 'Abdullah ibn Malik ibn Buhaynah: That the Messenger of Allah (may Allah bless him and grant him peace) went out for the morning prayer while a man was praying. The Messenger of Allah (may Allah bless him and grant him peace) struck his shoulder and said, "Do you intend to pray four [rak'ahs] or twice?"