Musannaf Ibn Abi Shayba 13:2409

Abu Bakr narrated to us, he said: Isma'il bin 'Ulayyah narrated to us, from Ibn Abi Najih, from Mujahid, regarding His saying: "And let not the witnesses refuse when they are called, and do not be weary." He said: If the testimony is with you, then you have been called.