Hadith.4284 - Hammad ibn Isa narrated from Abdullah ibn Maymun, from Abu Abdillah عليه السلام, who said: "A servant may make an exception (by saying 'If Allah سبحانه وتعالى wills') within forty days if he forgets. The Messenger of Allah سبحانه وتعالى (peace be upon him and his family) was visited by some Jews who asked Him صلى الله عليه وعلى آله وسلم about certain matters. He صلى الله عليه وعلى آله وسلم said to them: 'Come tomorrow, and I will inform you,' but He صلى الله عليه وعلى آله وسلم did not say 'If Allah سبحانه وتعالى wills.' So, Archangel Jibril عليه السلام was delayed from coming to Him صلى الله عليه وعلى آله وسلم for forty days. Then Jibril عليه السلام came and said: 'And do not say about anything: Surely I will do that tomorrow, except (by adding) If Allah سبحانه وتعالى wills. And remember your Lord عز وجل when you forget.' (Surah Al-Kahf 18:23-24)"