اذْهَبْ أَنتَ وَأَخُوكَ بِأيَاتِي
...
Go you and your brother with My Ayat,
This means with My proofs, evidences and miracles.
...
وَلاَ تَنِيَا فِي ذِكْرِي
And do not, you both, slacken and become weak in My remembrance.
Ali bin Abi Talhah related from Ibn Abbas that he said,
"This means do not be slow."
Mujahid reported that Ibn Abbas said,
"This means do not be weak."
The meaning here is that they should not slacken in the remembrance of Allah. Rather, they both should remember Allah during their meeting with Fir`awn so that the remembrance of Allah can be an aid for them against him. The remembrance of Allah would be their strength and their power that would defeat him.
Allah's statement;
اذْهَبَا إِلَى فِرْعَوْنَ إِنَّهُ طَغَى