Musannaf Ibn Abi Shayba 3:1479

Yazid ibn Harun narrated to us, from Ibn 'Awn, who said: I arrived in Medina and caught one rak'ah of the Isha prayer. I began to speak to myself, "How should I perform [the rest]?" I mentioned that to al-Qasim. He said: "Were you afraid that if you prayed four [rak'ahs], Allah would punish you?"