Sayyiduna Abdullah (bin) Abbas (may Allah be pleased with him) reported concerning this statement of Allah Almighty: "And remember your Lord when you forget." (Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said:) It refers to making the exception (saying in sha' Allah) when you remember. He said that this was specific to the Prophet (peace and blessings be upon him), and no one else has the right to make this exception, except that it may be done when it is joined to the oath.
Imam al-Tabarani recorded it in al-Mu'jam al-Awsat and al-Mu'jam al-Saghir; its chain contains a narrator, Abd al-Aziz bin Husayn, who is da'if (weak).