The companions of Rasulullah (sallallahu ‘alayhi wa sallam) noticed some grey strands of hair on the beard of Rasulullah (sallallahu ‘alayhi wasallam), and told him that you seem to be getting old very quickly. Nabiy (sallallahu ‘alayhi wasallam) replied that Surah Hud has made me old.
Sayyiduna ‘Abdullah ibn Abbas (radiyallahu ‘anhuma) says in my opinion it was this specific verse of Surah Hud, verse. 112:

…فَاسْتَقِمْ كَمَا أُمِرْتَ وَمَنْ تَابَ مَعَكَ وَلَا تَطْغَوْا
Remain steadfast as you have been commanded, [you] and those who have repented with you, and do not transgress

Here is a talk delivered by my beloved father, Moulana Haroon Abasoomar (rahimahullah) on the topic of remaining steadfast in all aspects of Islam.

Download here