Musannaf Ibn Abi Shayba 7:187

Abu Bakr narrated to us, he said: Abu al-Ahwas narrated to us, from Ibrahim ibn al-Muhajir, from Ibrahim, who said: If a man says, "I adjure you" (aqsamtu 'alayk), it is nothing. But if he says, "I adjure him by Allah" (aqsamtu 'alayhi bi-Allah), then it is an oath requiring expiation (kaffarat yamin).