ٱذْهَبْ أَنتَ وَأَخُوكَ بِـَٔايَٰتِى وَلَا تَنِيَا فِى ذِكْرِى · ٱذْهَبَآ إِلَىٰ فِرْعَوْنَ إِنَّهُۥ طَغَىٰ · فَقُولَا لَهُۥ قَوْلًا لَّيِّنًا لَّعَلَّهُۥ يَتَذَكَّرُ أَوْ يَخْشَىٰ
Idhhab anta wa akhūka bi-āyātī wa lā taniyā fī dhikrī · Idhhabā ilā Firʿawna innahu ṭaghā · Fa-qūlā lahu qawlan layyinan laʿallahu yatadhakkaru aw yakhshā
"'Go, you and your brother, with My signs, and do not slacken in remembering Me. · Go, the two of you, to Pharaoh; he has transgressed. · Speak to him a gentle word; perhaps he will take heed or will fear.'"
The root و-ن-ي — occurs here for the first time in the series. The sense the lexicographers give: to weaken, to slacken, to slow and come to a halt in a work. Wanā fi'l-amr — he went at the matter slowly. The word occurs only here in the Qurʾān.
I record this as my own reading, and its ground is the object of the prohibition: the measure given to the two messengers is not not to slacken in the commission — it is not to slacken in remembrance. That is, what is required to be continuous is not the work but that to which the work is attached.
And this overlaps with the ground of the request in the twenty-ninth verse (wa nadhkuraka kathīrā). What Moses gave as his ground is exactly what is commanded of him. This is an overlap that can be verified within the sūra.