Musnad Ahmad 696:1

Hajjaj ibn Muhammad narrated to us, he said: Layth, meaning Ibn Sa’d, said: Bakr, meaning Ibn Abdullah ibn al-Ashajj, narrated to me, from Nabil, the owner of the cloak, from Abdullah ibn Umar, from Suhayb, the companion of the Messenger of Allah, may Allah bless him and grant him peace, that he said: "I passed by the Messenger of Allah, may Allah bless him and grant him peace, while he was praying, so I greeted him. He returned [the greeting] to me with a gesture." And he said: "I do not know except that he said: A gesture with his finger."