Musannaf Ibn Abi Shayba 1:1814

Abu Bakr narrated to us, from Jarir, from Mughira, from Abu Ma'shar, who said: I asked Ibrahim about the *siwak* (tooth stick). He said: "And who can endure the *siwak*? They used to use the *siwak* after the *witr* (prayer), before the two *rak'ahs*."