It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that he held that the istithna' (saying 'in sha' Allah') is (valid and effective) even after a year. Then he recited this verse: "And never say of anything, 'Indeed, I will do that tomorrow,' except (adding), 'If Allah wills.' And remember your Lord when you forget." He said: "(That is,) when you remember."
Imam al-A'mash (may Allah have mercy on him) was asked: "Did you hear this report from Mujahid (may Allah have mercy on him)?" He replied: "Layth (may Allah have mercy on him) narrated it to me from Mujahid (may Allah have mercy on him)."
Imam al-Tabarani (may Allah have mercy on him) said: "Only Abu Mu'awiyah (may Allah have mercy on him) narrated this hadith from al-A'mash (may Allah have mercy on him), and Yahya (may Allah have mercy on him) is alone in transmitting it."
Hadith Referenceالمعجم الأوسط / باب الألف / 119
Hadith Takhrij«قال الطبراني: فقيل للأعمش سمعت هذا من مجاهد فقال حدثني به الليث عن مجاهد «المعجم الأوسط، ج 1، ص 44»» «أخرجه البيهقي فى «السنن الكبرى» برقم: 19989، والطبراني فى «الكبير» برقم: 11069»