Allah says in surah Ghafir:
فَاصْبِرْ إِنَّ وَعْدَ اللَّـهِ حَقٌّ وَاسْتَغْفِرْ لِذَنبِكَ وَسَبِّحْ بِحَمْدِ رَبِّكَ بِالْعَشِيِّ وَالْإِبْكَارِ
So be patient; the promise of Allah is certainly true. And seek forgiveness for your sins, and exalt the praises of your Lord in the last part of the day and the first part of the day. [40:55]
Sheikh ‘Abd al-Rahman ibn Naasir al-Sa’di had the following beautiful words of commentary on this ayah:
ـ { فَاصْبِرْ } يا أيها الرسول كما صبر من قبلك من أولي العزم المرسلين. { إِنَّ وَعْدَ اللَّهِ حَقٌّ } أي: ليس مشكوكًا فيه، أو فيه ريب أو كذب، حتى يعسر عليك الصبر، وإنما هو الحق المحض، والهدى الصرف، الذي يصبر عليه الصابرون، ويجتهد في التمسك به أهل البصائر. ـ
فَاصْبِرْ
So be patient …
O Messenger, just as those messengers of great determination before you exercised steadfast patience.
إِنَّ وَعْدَ اللَّهِ حَقٌّ
The promise of Allah is certainly true.
i.e. there is no confusion or doubt about it, nor is it a false claim, all of which would make it difficult for you to be patient. On the contrary, it is the absolute truth and pure guidance which enables the patient ones to be patient and which the people of insight strive to adhere to.