Answer
It is a well-known and acceptable fact that the Qur’aan and Hadith does not contain express ruling for each and every matter. Many rulings of Shari?ah are deduced from the Qur’aan and Hadith based on principles as explained by the Fuqahaa (jurists).
It is therefore unprincipled to request for a Hadith in support of writing down intentions during Tashkeel. The act is Mubaah (permissible) and it suffices that there is no prohibition for that, on condition it is not regarded as compulsory. Writing down intentions during Tashkeel is, at most, an administrative issue.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai