It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he said: "'And never say of anything: I shall do that tomorrow, except (adding): if Allah wills; and remember your Lord when you forget' means that you should say 'In sha Allah' (if Allah wills)."
Hadith Referenceالمعجم الأوسط / باب الألف / 930
Hadith Takhrij«أخرجه الطبراني فى «الكبير» برقم: 12817»