Musnad Ahmad 907:222

Waki’ narrated to us, Mis’ar narrated to us, from ‘Ubayd Allah ibn al-Qibtiyya, from Jabir ibn Samura, who said: When we prayed behind the Messenger of Allah (peace and blessings of Allah be upon him), one of us would gesture to his brother on his right and on his left. When the Messenger of Allah (peace and blessings of Allah be upon him) finished praying, he said: "What is the matter with one of you doing this, as if they were the tails of restive horses? It is sufficient for one of you to say like this"—and he placed his right hand on his thigh and gestured with his finger—"then he should greet his brother on his right and on his left."