In al-Mu'jam al-Kabir this narration is reported from the same Companion: "The witr is a right; whoever wishes may perform seven rak'ahs of witr." After this the narrator mentioned the narration as before.
(Allamah al-Haythami says:) I say: Its chain contains a narrator, Ash'ath bin Sawwar, whom Imam Ahmad and a group declared da'if (weak), while Yahya bin Ma'in called him thiqah (trustworthy).
Imam Abu Dawud has also recorded this narration, but he did not record these words: "Whoever is not able to perform one rak'ah of witr, let him perform it by gestures." (Allamah al-Haythami says:) I say: The narration of Imam Ahmad concerning the number of rak'ahs of witr will come later, if Allah wills.