Al-Kāfi

Hadith 112
5ـ عَلِيٌّ بِإِسْنَادِهِ قَالَ أَدْنَى السِّوَاكِ أَنْ تَدْلُكَ بِإِصْبَعِكَ.
5. Ali through his chain of narrators has narrated the following: “He (the Imam) (a.s.), has said, ‘The least for brushing is using one’s fingers as a tooth brush.”
Source: Shaykh Muḥammad b. Yaʿqūb al-Kulaynī