The fifth oath is:

وَالسَّمَاءِ وَمَا بَنَاهَا (and by the sky, and the One who built it...91:5). Most clearly the context suggests that the ma may be taken as infinitival particle [ masdariyyah ], signifying 'by the sky and its make ' as elsewhere in the Qur'an غَفَرَ‌ لِي رَ‌بِّي '...how my Lord has forgiven me [ 36:27] ' (1)

1. It should be kept in mind that the translation of the text is not based on this construction. It is based on taking 'ma' as mousulah in the sense of 'the one' as adopted by Maulana Thanawi, and explained by the author in the following verse. word taswiyah means, to proportion, balance, perfect. The meaning of this word has been explained in the previous Surah.