Tafsir of Al-Ma'aarij 70:33

They are established in them with justice, neither denying them nor any part of them, nor concealing them, out of respect for the rights of people regarding what pertains to them, and in glorification of the command of Allah, the Exalted and Majestic, regarding what pertains to His rights—glory be to Him. Some have specified the testimony to that which relates to the rights of servants, noting that it is subsumed under "trusts," but it was singled out for mention to demonstrate its virtue. It was put in the plural form due to the diversity of its types; had this not been considered, it would have been singular—as has been said—because it is a verbal noun (masdar) that encompasses both the few and the many. The majority of reciters read it in the singular form, as you have heard just now.