Mujahid (may Allah have mercy on him) narrates from Sayyiduna Ibn Abbas (may Allah be pleased with them both) concerning the saying of Allah, the Mighty and Majestic: "And remember your Lord when you forget" — that he said: "When you forget to make the exception (by saying in sha' Allah), then make the exception whenever you remember."
He said: "This (concession) was specifically for the Messenger of Allah (peace and blessings be upon him); it is not permissible for any of us to make the exception unless it is joined (i.e. connected) to his oath."
Imam al-Tabarani (may Allah have mercy on him) said: "No one narrated this hadith from Ibn Abi Najih except Abd al-Aziz bin al-Husayn, and al-Walid bin Muslim is alone in narrating it."
Hadith Referenceالمعجم الأوسط / باب الميم / 6872
Hadith Takhrij«أخرجه الحاكم فى «المستدرك» برقم: 7928، والطبراني فى «الكبير» برقم: 11143، والطبراني فى «الصغير» برقم: 876»