Musannaf Ibn Abi Shayba 20:71

Shababa narrated to us, he said: Ibn Abi Dhi'b narrated to us, from Abu Jabir al-Bayadi, from Sa'id ibn al-Musayyib, he said: The Messenger of Allah (may Allah bless him and grant him peace) gave a judgment regarding a man who changes his testimony. He said: "He is to be held to the first [testimony]."