Musannaf Ibn Abi Shayba 3:3559

Waki' narrated to us, he said: Jarir ibn Hazim narrated to us, from Isa ibn Asim al-Asadi, from Sa'id ibn Jubayr, who said: If I were to leave, or if the two rak'ahs after the Maghrib prayer were left, I would fear that I would not be forgiven.