Musannaf Ibn Abi Shayba 13:2414

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Muhammad ibn Thabit narrated to us, he said: I heard ‘Ata’, and he was asked about [the verse]: "And let not the witnesses refuse when they are called upon," before they have testified or after? He said: No, rather after they have testified.