To what extent is the narration from Tawus authentic: "Indeed, the dead are tried in their graves for seven [days], so they (the Companions/early Muslims) used to recommend that food be given on their behalf during those days," which is used by Sufis as evidence for the desirability of giving food on behalf of the deceased?

Conclusion: The narrations concerning the trial of the dead in their graves for seven days are three: the narration of Tawus al-Yamani, whose chain of transmission is sound; the narration of Ubaid bin Umair, which contains weakness; and the narration of Mujahid, whose chain of transmission has not been traced. Some scholars used these narrations as evidence that the duration of the trial for believers in their graves is seven days, considering them mursalat (narrations with a missing link in the chain of transmission) that gain strength by their جمع (collective weight) and are treated as marfu' (attributed to the Prophet) because they pertain to matters of the unseen. However, this assertion by Al-Suyuti is weak because the narration of Tawus is the only one whose chain of transmission is sound, and it is contradicted by authentic prophetic hadiths that describe the trial of the grave as occurring once. Furthermore, there is no mention in the authoritative books of jurisprudence that states the desirability of feeding on behalf of the deceased for seven consecutive days.