Musannaf Ibn Abi Shayba 3:2384

Waki’ narrated to us, from Shu’bah, who said: I asked al-Hakam and Hammad about a man who remembered a prayer while he was in [another] prayer. They both said: If he remembers it before he performs the tashahhud or sits for the duration of the tashahhud, he should abandon this one and return to that one. If he remembers it after that, he should count this one [as valid] and return to that one.