Tafsir of Al-Ma'aarij 70:33

And those who stand firm in their testimonies. This means that they guard their testimonies. They do not add or decrease from what they testify to nor do they conceal their testimonies. Allah says in another Ayah,

ومن يكتمها فٳنه ءاثم قلبه

Who hides it, surely, his heart is sinful. (2:283) Then Allah says,

والذين هم على صﻼتهم يحافظون