Tafsir of Al-Adiyat 100:7

(And indeed he), meaning man, as stated by al-Hasan and Muhammad ibn Ka‘b, (is to that), meaning to his ingratitude, (a witness), because its effect is apparent upon him. Thus, the witnessing is through the tongue of state, which is more eloquent than the tongue of speech.

It has been said that it is through the tongue of speech, but in the Hereafter.

It has also been said that "witness" (shahid) is derived from being present (shuhud) rather than from testifying (shahada); meaning that he is ungrateful while aware of his ingratitude, and commits evil while aware of it, which is the ultimate level of blameworthiness. The first [interpretation] is the most apparent.

Ibn Abbas and Qatada stated that the pronoun in "indeed he" refers to Allah, the Exalted. That is: indeed His Lord, may He be glorified, is a witness over him; thus, the statement is by way of a threat. Al-Tabrizi chose this view, stating it is the most correct because a pronoun must refer to the nearest mentioned antecedent.

However, it is argued that such an obligation is not absolute. Furthermore, the coherence of the pronouns and their lack of fragmentation support the first view, for the preceding pronoun—namely, the pronoun in "for his Lord" (li-rabbihi)—necessarily refers to man, as does the subsequent pronoun, namely the pronoun in His saying: