مَّا تَسْبِقُ مِنْ أُمَّةٍ أَجَلَهَا وَمَا يَسْتَـْٔخِرُونَ
Maa tasbiqu min ummatin ajalahaa wa maa yastaakhiroon
No community can outrun its appointed time, nor can they delay it.
Ajal names the term appointed to a life, and here the Qur'an sets it on a whole people, which shifts the frame from one person's mortality to the lifespan of a civilisation. Sabaqa is a word from racing, to get out ahead, so the picture is of a nation trying to outrun a schedule it cannot see. Coming immediately before the taunt that the Prophet is mad, the assurance is aimed at the one being taunted.