And this surely is great evidence if you can understand --
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Tafseer
'Abdullāh Ibn 'Abbās / Muḥammad al-Fīrūzabādī
تفسير : (and lo! that) i.e. the qur'an (verily is a tremendous oath, if ye but knew) if you but believe.
Jalāl al-Dīn al-Maḥallī
تفسير : and indeed it, the swearing by these [setting-places of stars], is a tremendous oath, if you only knew — if you were people of [deep] knowledge, you would realise the magnitude of this oath.