Majma al-Zawaid — Hadith 11148

It is reported about Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) that he held that the exception (saying In sha' Allah) is valid even if it comes a year later. Then he recited this verse: "And never say of anything, 'I will do that tomorrow,' unless you say In sha' Allah, for then the ruling is different; and remember your Lord when you forget." Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: (It means) when you remember (you may make the exception).
Al-Tabarani recorded it in al-Mu'jam al-Awsat and al-Mu'jam al-Kabir, and its narrators are thiqat (trustworthy).