Is there any other testimony of Holy Scripture regarding the intercessory prayer of the saints?

Indeed, the book of Revelation shows the ministry of both the martyrs and the angels before the throne: “I saw under the altar the souls of those who had been slain for the word of God… they cried out with a loud voice, ‘Sovereign Lord, holy and true, how long…?’” (Rev 6:9–10; see Q167, Q346); “Another angel came and stood at the altar with a golden censer. Much incense was given to him, so that the angel should add it to the prayers of all the saints on the golden altar which was before the throne. The smoke of the incense, with the prayers of the saints, went up before God from the angel’s hand” (Rev 8:3–4; see Q100, Q402).

It is therefore established that the faithful departed intercede, and that the angels are given a ministry as messengers and “fellow servants” (Rev 22:9; see Q402), “sent out to serve for the sake of those who will inherit salvation” (Heb 1:14; see Q100, Q107).