Musannaf Ibn Abi Shaybah

Hazrat Ata, Hazrat Tawus, Hazrat Mujahid, Hazrat Nakha'i, and Hazrat Zuhri (may Allah have mercy on them) say that if a man says to his wife, "If I do not do such and such, then, Insha'Allah, you are divorced," then his exception will be considered valid.